Robot palletiser light curtain: safety distance, performance level, and a deliberate defeat
A robot palletiser handles 20 kg bags at up to 1,100 cycles/h. Operators load empty pallets through a 1,200 mm wide opening protected by a safety light curtain with 30 mm detection capability. The curtain output goes to a single-channel safety relay that de-energises two contactors in series in the robot drive circuit; there is no feedback (EDM) contact and no self-test. The robot controller's stopping time, measured on a new machine with a light load, is 0.45 s; the curtain response time is 0.032 s and the relay drop-out is 0.030 s. The distance from the curtain plane to the nearest reachable hazard is 1,000 mm. The cell risk assessment calls for Performance Level d to ISO 13849-1. During a busy shift the team leader taped cardboard over the lower 400 mm of the curtain so pallets could pass faster, and an operator then reached in to clear a jammed bag while the robot was still moving; a near miss followed. The site asks for your assessment before restart.
1. Calculate the minimum safety distance per ISO 13855 and decide whether the installed 1,000 mm is acceptable. Show how sensitive the answer is to the stopping time.
Read the worked answer
Response time. T = 0.45 (machine) + 0.032 (curtain) + 0.030 (relay) = 0.512 s.
Intrusion allowance. For d = 30 mm (≤ 40 mm): C = 8(d − 14) = 8 × 16 = 128 mm.
Distance. First try K = 2,000 mm/s: S = 2,000 × 0.512 + 128 = 1,152 mm. Since this exceeds 500 mm, ISO 13855 requires recalculation with K = 1,600 mm/s: S = 1,600 × 0.512 + 128 = 947 mm (and not less than 500 mm).
Verdict on paper: 947 mm required versus 1,000 mm installed - only 53 mm of margin, about 5 %. That is inside measurement uncertainty and inside any brake wear.
Sensitivity. The 0.45 s was measured on a new machine with a light load. Recompute with a realistic 20 % brake-wear/load allowance, T = 0.602 s: S = 1,600 × 0.602 + 128 = 1,091 mm - the installation is then 91 mm short. Invert the formula: the greatest stop time the current 1,000 mm can tolerate is T_max = (1,000 − 128)/1,600 = 0.545 s, leaving only 0.483 s for the machine against a measured 0.45 s. A 7 % allowance for brake wear, load and speed variation is not defensible.
Conclusion: the distance cannot be accepted as-is. Re-measure stopping performance at maximum rated load and maximum speed with the brake at its wear limit, then either move the curtain back to at least 1,100 mm or add mechanical restriction so the reach-through path is longer.
2. Address the defeated curtain and the behaviour. What combination of engineering and management controls do you require, and what do you refuse to accept?
Read the worked answer
Start from the design, not the person. Taping over a curtain is predictable whenever the guard blocks a task that must be done every shift. The cell requires pallet loading and jam clearing behind the curtain plane, so the design has built in an incentive to defeat. Disciplinary action or 'refresher training' alone is not a control and will not survive the next busy shift.
Hierarchy of controls, in order:
- Eliminate the access: automatic pallet dispenser/magazine or a two-position turntable so pallets are loaded from outside the hazard zone.
- Engineering: fixed guarding with an interlocked, guard-locked gate for loading; a dedicated, interlocked jam-clearing door with a request-to-enter sequence that puts the robot in a safe state; safe limited speed inside the muting zone; clear floor markings and a restart warning.
- Administrative: a written jam-clearing procedure using lockout/tagout or the safe-state function, not reaching in; permit-based entry; verification of the safe state before entry.
- Behavioural: training and communication are supporting measures only.
What I will not accept: restarting the cell on a promise to retrain, or on the cardboard being removed without a change to the design. A near miss with a moving robot is a serious warning; the same defect almost certainly exists on sister cells, so a site-wide check for defeated devices (taped curtains, wire-bridged interlocks, removed keys) is required.
Process. Record the assessment in writing, raise a non-conformance against the risk assessment, inform the line manager and safety manager, and do not sign off the cell as safe until the safety functions are restored, validated and the entering procedure is physically enforced.