Alignment issues / false trips / strong interference
Ambient light, welding arc, reflective surfaces. A practical, shop-floor guide to diagnosing and fixing safety light curtain misalignment, nuisance trips, and electro-optical interference. Includes alignment routine, optical/mechanical/EMI countermeasures, and an acceptance log for audits.
Baseline: Treat every unexplained stop as a real stop until proven otherwise. Verify the safety chain (curtain → safety controller/relay → contactors K1/K2) before resuming automatic operation.
1) Symptoms → likely causes
| Symptom | Likely cause | First checks |
|---|---|---|
| OSSD drops intermittently; no object visible | Receiver saturation by sunlight/welding arc, mirror reflections, EMI on lines | Check bargraph/diagnostic LED; toggle lights/arc; shield or shade and retest |
| Low alignment margin on certain heights | Mechanical twist, bowed frame, loose brackets | Measure diagonal distances; re-shim; verify straightness with string/laser |
| Trips when drives start/stop | EMI from VFD/servo, poor earthing, cable routing parallel to power | Scope 24 V line; separate harness; add ferrites; single-point ground |
| Trips near mirror-like surfaces | Specular reflections creating ghost beam paths | Tilt TX/RX by 3–5°; apply matte film/paint to mirrors; add side hoods |
2) Alignment routine (repeatable)
Procedure
- Mounting — loosen brackets; set both sticks to mid-slot; verify same elevation datum.
- Coarse align — use the receiver bargraph to maximize signal; center TX/RX parallel.
- Margin test — cover top/bottom few beams with a 10–20 mm card; ensure the margin ≥ 3 dB (or “≥2 LEDs spare”).
- Lock-down — tighten to torque; re-check after torque to catch twist.
- Vibration — run the machine; watch bargraph flicker; add stiffeners if needed.
ASCII layout
TX |=================| RX
^ tilt 3–5° off parallel when facing shiny or glass surfaces
Keep TX/RX clear of columns; avoid mounting on flimsy guards3) Interference & how to eliminate it
A. Ambient light & welding arc
- Add shrouds/hoods or sun-shades; avoid skylight lines of sight.
- Angle TX/RX 3–5° so direct/retro reflection cannot re-enter the receiver.
- Use welding curtains or dark matte panels between arc and receiver.
- Fit window covers (sacrificial, anti-spatter) and clean on schedule.
- Move auxiliary IR sources (heaters, sensors) out of beam plane.
B. Reflective floors/walls & mirrors
- Rotate columns slightly; ensure no beam can bounce into RX.
- Matte tape/paint on polished edges; anti-glare films on glass.
- Increase separation to walls or install side baffles.
C. EMI/Noise from power electronics
- Route signal harnesses ≥ 200 mm from motor/VFD cables; cross at 90°.
- Star-point 0 V at the safety controller; bond PE to machine frame.
- Shielded cables: drain grounded one end (controller side) to avoid loops.
- Ferrite cores near RX/TX and controller; add TVS on 24 V if spikes seen.
- Separate fusing for OSSD1/OSSD2; avoid shared returns with inductive loads.
4) Mechanical stability (often overlooked)
- Use stiff brackets with two-point mounting top & bottom; add mid-span support for >1.2 m heights.
- Verify frame squareness; shim to remove twist; avoid mounting on flexible doors/panels.
- For presses with shock: isolate with rubber pads but keep relative TX/RX coplanar.
5) Acceptance worksheet (copy/paste)
Machine: ____________________ Area: ____________________
Protective height (mm): ______ Resolution (mm): 10 / 14 / 30 / 40
Alignment margin (LED/dB): ____/____ Tilt (deg): ____
Reflective risks handled: ☐ yes ☐ n/a Shields/hoods installed: ☐
EMI controls: cable sep >=200 mm ☐ 90° crossings ☐ shield drain one end ☐
Response times measured (ms): Curtain ___ Relay ___ K1 ___ K2 ___ T_total=___
Safety distance check S = K×T_total + C Verified: ☐ yes
Photos archived: TX, RX, wiring, shields, surroundings ☐6) Common mistakes & fast fixes
| Mistake | Symptom | Fix |
|---|---|---|
| Parallel routing with motor leads | Trips when motors start | Re-route, 200 mm spacing, 90° crossings, ferrites |
| Mounting perfectly parallel to glass/steel | Ghost trips in sunshine | Tilt 3–5°, matte treatment, side baffles |
| Shared return with coils/solenoids | Random OSSD drop | Separate returns; flyback diodes/RC across coils |
| No cleaning plan in oily/welding areas | Gradual margin loss | Window cover + cleaning schedule + spare covers |
7) Troubleshooting flow (10-minute rule)
- Observe: read LEDs/diagnostics; note time vs machine actions.
- Isolate optics: shade with cardboard; if trip disappears → optical source.
- Isolate EMI: power drives off; inject dummy load; watch 24 V ripple (scope).
- Re-align: maximize bargraph; confirm ≥2 spare LEDs across whole height.
- Record: fill worksheet; photo evidence; add mitigations; re-test.
8) Quarterly test log (CSV)
Item,Test,Expected,Actual,Result,Notes
1,Alignment margin,>= 2 LEDs spare across height,,,
2,Optical shielding,No trip with arc/light on,,,
3,EMI immunity,No trip during motor/VFD start/stop,,,
4,Cleanliness,Windows clean; covers intact,,,
5,Records,Worksheet + photos archived,,,
Keep logs alongside ISO 13855 distance math and safety controller parameter exports.