Nuisance Stops at the Collaborative Palletizer
A Tier-1 automotive supplier installed a UR10e cobot on a palletizing cell. A safety laser scanner defines two zones: when an operator enters the outer zone, the robot slows to 60% speed; when the inner zone is breached, a Category 2 stop occurs. The cell target is 12 cycles per minute, but actual output is 9.5. Operators record 37 nuisance stops per 10-hour shift, each with about 45 seconds of recovery. The original risk assessment was completed before a gravity roller conveyor was added 400 mm inside the outer zone boundary. The line leader wants to mute the scanner during automatic cycles. Maintenance proposes increasing the stop time delay from 200 ms to 500 ms. The controls engineer reports a 12 ms PLC scan and a 180 ms safety response time.
1. Assess the line leader's muting proposal and maintenance's stop-delay proposal. What safety and production risks does each introduce?
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Muting is acceptable only for material passage and only with a designed, validated muting function using sensors that detect the material, not people. Muting the scanner during automatic cycles would remove the safeguarding that keeps operators out of the robot envelope during the highest-risk phase. That is not a tuning change; it invalidates the risk assessment and likely the required performance level.
Increasing the stop delay from 200 ms to 500 ms increases travel before the stop completes. If the approach speed at the outer boundary is 1.2 m/s, an extra 300 ms adds about 360 mm of travel. The conveyor already intrudes 400 mm into the outer zone, so the protective distance may no longer be sufficient. The change also alters the validated safety function unless the stopping performance and separation distance are recalculated and revalidated.
Both proposals treat a layout and cycle-time problem as a guarding problem. The likely finding is that the conveyor encroachment causes false protective-zone entries and that robot speed and path, not the scanner, are the real cycle-time constraints.
2. Recommend a compliant way to recover cycle time. Give the engineering and validation sequence, including what you would measure.
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Start by updating the risk assessment for the new conveyor and mapping the actual operator access paths. Move or reorient the conveyor and scanner so the outer warning zone is outside normal working positions, and create a dedicated load station outside the robot envelope.
Then implement safety-rated speed and separation monitoring. Tie scanner zone distance to a validated robot speed limit and stopping distance. Use the outer zone to slow the robot only when a person is approaching, and the inner zone to stop. Do not mute; do not stretch the stop delay unless the stopping-distance calculation supports it.
Recover cycle time through path optimization, pick-and-place sequencing, gripper design, and pallet pattern changes. Measure actual stopping time with a calibrated test, confirm separation distances, verify PLC and safety response timing, and validate per the applicable robot and collaborative-system standards. Run a shift with recorded stops to prove the nuisance rate falls before releasing the change.