Why Does a Cylinder Magnetic Sensor Switch Twice or Lose the End Signal?

A cylinder magnetic sensor that switches twice on one piston pass is usually seeing a magnet-field structure with two actuation zones — the piston ring's field has leading and trailing edges, and a sensor positioned where both edges cross it produces two pulses — or it is responding to vibration/other magnetic fields near the trip point. A lost end signal usually means the sensor sits where the piston magnet's field is too weak (position error), the piston physically does not reach the sensing zone (stroke/cushioning issue), or the speed is too high for the input chain to register. The first safe check is positional: with the machine at rest, hand-move the piston (or jog per site procedure) and map exactly where the sensor switches along the stroke. All checks near moving machinery are stopped-machine work.

Key Takeaways

A piston magnet has a field profile, not a point. The ring magnet produces a detectable zone with edges; where the sensor sits inside that zone determines single, double or lost switching. Position is the application.
Double switching is classic edge-crossing. Both field edges crossing the sensor's hysteresis band produce two pulses per pass; moving the sensor fully inside (or outside) the zone — or selecting appropriate hysteresis — resolves it.
Lost end signal is usually geometry. The sensor sits beyond the magnet's reach at full extension, the piston does not fully stroke (cushioning, mechanical stop), or the magnet has weakened — each has a distinct check.
Speed interacts with the input chain. A fast piston can produce a shorter ON-time than the PLC input filter or scan accepts; the sensor switched — the logic missed it.
Nearby magnetism and vibration fake both symptoms. Welding fixtures, adjacent cylinders' magnets and bracket vibration move the sensor's effective trip point; correlation with machine state identifies them.

Symptom / Likely Cause / Safe Check Table

Symptom	Likely cause	Safe check
Two pulses per piston pass	Sensor positioned where both edges of the magnet zone cross the hysteresis band; hysteresis too narrow for the zone	Hand-cycle (machine at rest) and mark both switch points along the stroke; compare their separation with the sensor's zone behavior
End signal missing entirely	Sensor beyond the magnet's detectable zone at full stroke; piston not reaching full stroke; weakened/damaged magnet	Slowly hand-stroke the piston to full extension and watch the indicator; mark the switch zone vs. sensor position; verify piston reaches the mechanical end
Signal lost only at high speed	ON-time shorter than PLC input filter/scan accepts	Compare pulse duration at operating speed with the input filter/scan specification; observe indicator vs. PLC tag (indicator shows the switch the logic missed)
Chatter at the trip point	Vibration moving sensor or field across the threshold; nearby magnetic field	Correlate with machine vibration state; check for adjacent magnets (other cylinders, fixtures) within influence distance
Worked before, fails after service	Sensor re-mounted at a different position; different cylinder (different magnet); bracket change	Compare current mounting position against commissioning marks; verify cylinder model unchanged
Intermittent loss, worse with motion	Cable/connector flexing at the cylinder (the classic moving-point failure)	Inspect cable at the moving point for damage; wiggle-test by qualified personnel with logic monitored
Both end sensors behave oddly	External magnetic source or shared-path electrical fault	Map other magnetic sources near the cylinder; check both sensors' common path

Safety Boundary

Safe observations: with the machine stopped and locked out, hand-stroking or jogging the piston per the site's procedure while watching the sensor indicator; marking switch positions; visual inspection of mounting, cable and cylinder.

Qualified-person work: any electrical verification (continuity, supply, wiggle-testing with power), PLC input configuration changes, and any work requiring the machine to move under power — those need the site's procedures and, where relevant, personnel clear of moving parts. Pneumatic systems store energy: follow lockout/vent procedures before touching the mechanism.

Inputs to Collect Before Changing Settings

Cylinder brand and model, bore, stroke, and whether the piston magnet is confirmed (datasheet or previous operation);
Sensor model and mounting type (groove / round-body tie-rod format / other per the installed type), with its hysteresis/switching-point behavior from the data sheet;
Measured switch zone: where along the stroke the sensor switches and releases (hand-cycle marks);
Operating piston speed and required signal duration in the logic;
Nearby magnetic sources: adjacent cylinders, magnetic fixtures, welding equipment;
PLC input filter/scan settings for the channel;
Fault history: since when, after what change, at what speeds.

Diagnostic Sequence (Least Invasive to Specialized)

Observation: double switching on each pass. Hypothesis: both edges of the magnet zone cross the sensor. Check: hand-cycle at rest; mark both switch points and the zone extent. Expected result: two distinct switch positions close together within the magnet zone. Next action: reposition the sensor fully inside the zone (or per the application's required signal point) so a single continuous ON results; verify at operating speed.
Observation: end signal missing. Hypothesis: sensor outside the detectable zone at full stroke. Check: hand-stroke fully; note the furthest switch position vs. the sensor location. Expected result: the magnet's zone ends before the sensor position. Next action: move the sensor into the zone at the true end position; re-verify.
Observation: end signal missing even with correct zone overlap. Hypothesis: piston does not reach full stroke (cushioning set too hard, mechanical stop, pressure low). Check: verify piston position at full extend against the mechanical end (visual/mechanical, machine locked out). Expected result: piston stops short of the zone. Next action: correct the pneumatic/mechanical cause — the sensor is honest.
Observation: signal present at slow speed, absent at operating speed. Hypothesis: ON-time below the input chain's resolution. Check: estimate pulse duration from piston speed and zone length; compare with PLC input filter/scan. Expected result: duration shorter than the logic accepts. Next action: adjust input filter/scan within the logic design, or extend the detectable zone (position/sensor choice); the indicator-vs-PLC comparison confirms it (see the pulse-chain method in speed-sensor pulse troubleshooting guide).
Observation: chatter or double-switching correlating with machine vibration. Hypothesis: vibration or external magnetic field near the trip point. Check: correlate with machine state; survey adjacent magnets (other cylinders, fixtures) within influence distance. Expected result: symptom appears only with the vibration source or specific adjacent cylinder active. Next action: stiffen mounting or move away from the external field; re-verify.
Observation: intermittent faults with motion. Hypothesis: cable/connector flex failure at the moving point. Check: qualified-personnel wiggle test with logic monitored; inspect the cable at the flex point. Expected result: fault reproduced by specific cable positions. Next action: replace the cable/connector with the correct part (the replacement-equivalence discipline is in cable-or-bracket replacement topic).
Observation: symptoms since a cylinder or sensor replacement. Hypothesis: different magnet strength/position (new cylinder) or different sensor model behavior. Check: compare cylinder and sensor models old vs. new against commissioning records. Expected result: a substitution explains the change. Next action: re-run steps 1–2 for the new hardware; the selection-side guidance is in magnetic-vs-inductive cylinder comparison.
Observation: all checks pass, fault persists. Hypothesis: electrical path or input problem. Check: qualified personnel verify supply/output at the sensor and the input channel. Expected result: path fault isolated. Next action: route to the PLC input guide (intermittent-PLC-input troubleshooting guide).

Conditional Corrective Actions

Double switching confirmed as edge-crossing → reposition the sensor inside the zone; if the application needs the signal exactly at an edge position, discuss a model with suitable hysteresis behavior with support.
Piston not reaching the zone → fix the pneumatic/mechanical cause (cushioning, stops, pressure) — do not move the sensor to a "convenient" position that no longer confirms the end.
Speed/pulse-width mismatch → correct the input chain (filter/scan) or zone length; never accept a "sometimes-missed" end signal in sequencing logic.
External field influence → increase distance or reorient; verify adjacent-cylinder layouts in multi-cylinder stations.
Flex-point cable failure → replace with the specified cable and restore strain relief per the machine design.

Replacement and Reselection Criteria

Reselect when: the cylinder's magnet/sensor combination cannot produce the required signal at the required position and speed; the environment demands a variant family (welding areas generate fields and spatter — welding-area versions exist; hazardous areas require the explosion-proof variant, selected against the area classification); or the mounting format does not fit the cylinder (the format matching — groove, tie-rod formats — is selection guidance in magnetic-vs-inductive cylinder comparison). For end-of-travel applications where magnetic sensing fundamentally cannot meet the requirement, the position/limit-detection alternatives are compared in magnetic-vs-inductive cylinder comparison and travel-detection comparison.

Information to Send KJT Sensors Support

Cylinder brand/model, bore and stroke; piston-magnet confirmation;
Sensor model and mounting type; mounting position photos with the switch-zone marks;
Hand-cycle switch-zone measurement (switch point, release point, zone length);
Operating piston speed and required logic behavior (minimum ON-time);
Nearby magnetic sources description; vibration state correlation;
PLC input filter/scan settings; fault history and what changed.

Limitations

Switching point, hysteresis and zone behavior are model- and cylinder-specific: no numeric values are given; verify on the actual cylinder (KJT Sensors' magnetic sensors are stated to detect permanent magnets of all strengths on all common cylinder pistons — manufacturer-stated; the installed reality is verified on the machine).
This article covers installed-sensor diagnosis; choosing magnetic vs. inductive for a new application is owned by C03 (magnetic-vs-inductive cylinder comparison).

Frequently Asked Questions

Why does a cylinder magnetic sensor switch twice?

Because the piston magnet's field forms a zone with two edges, and the sensor sits where both edges cross its hysteresis band — one switch on entry, one on exit, with a brief release between them. Hand-cycle the piston at rest, mark both switch points, and reposition the sensor so the pass produces one continuous signal. If the application genuinely needs a signal at the edge, the sensor's hysteresis behavior becomes the selection variable — discuss it with support.

Why is the cylinder end-position signal sometimes lost?

Three families: geometry (sensor beyond the magnet's zone, or the piston stopping short of full stroke — cushioning, stops, pressure), speed (ON-time shorter than the PLC input chain accepts — the indicator switches, the logic misses), and path (flexing cable at the moving point, connector faults). The hand-cycle map distinguishes geometry; the indicator-vs-PLC comparison distinguishes speed; the wiggle test distinguishes path.

How do sensor position and piston speed affect switching?

Position decides which part of the magnet zone the sensor sees — inside it, one continuous signal; straddling an edge, double switching; outside it, nothing. Speed decides how long the signal lasts: zone length divided by piston speed gives ON-time, which must exceed what the PLC input filter and scan can register. A slow hand-cycle proves the sensor; operating speed proves the chain — test both.

Conclusion

Map the magnet's switch zone by hand-cycle before theorizing: position fixes double switching and most lost signals; piston stroke and cushioning explain the rest of the geometry cases; speed and the input chain explain "sensor works, logic misses"; flex-point cables explain intermittence. Reposition, correct the mechanics, match the chain, and only then consider a different sensor — with the format and environment variants selected through C03's guidance.

Sources

KJT Sensors, Magnetic Sensor category (groove, round-body tie-rod and related mounting formats; wear-free, short-circuit-proof, IP67; welding-area, explosion-proof and analog-output versions; detects permanent magnets of all strengths on all common cylinder pistons — manufacturer-stated; verified 2026-10-10) — https://www.kjt-sensors.com/list-cgycgq.html
KJT Sensors, Service & Support — https://www.kjt-sensors.com/list-services.html (verified 2026-10-10)
KJT Sensors internal knowledge base: magnetic and Hall-effect sensors (selection inputs: cylinder brand, groove type, magnet condition) — version current at drafting
General pneumatic position-sensing practice (magnet zone behavior, hysteresis, cushioning effects; no third-party claims)

Send Cylinder and Sensor Details

Provide the cylinder model, sensor model and mounting type, the hand-cycle switch-zone measurement, and the operating speed with required logic behavior via the KJT Sensors service page — application engineers will verify the position/hysteresis logic or recommend the matching variant.

https://www.kjt-sensors.com/
KJT Sensors