Safety Laser Scanner Protective Distance
A robot cell uses a horizontal safety laser scanner to protect the approach to a 6-axis robot. The scanner has a response time of 60 ms, the safety relay adds 20 ms, and the robot's worst-case stopping time is 250 ms. The scanner's detection plane is set at 300 mm above the floor, and the cell's risk assessment uses a constant C = 1200 − 0.4H mm, with H the detection height, and an approach speed K = 1600 mm/s. The scanner is mounted so that its protective field boundary is 1.30 m from the nearest hazard point. Operators sometimes walk directly toward the robot at about 1.6 m/s. The scanner's field is configured with a 100 ms muting during pallet exchange, and the muting is triggered by a single sensor. The robot controller's stop category is 1 (controlled stop). A recent near miss occurred when an operator entered the field during muting.
1. Calculate the minimum protective distance using S = K(T) + C and compare it with the installed 1.30 m.
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Total response time T = scanner 60 ms + safety relay 20 ms + robot stop 250 ms = 330 ms = 0.33 s. The approach term is K × T = 1600 × 0.33 = 528 mm. For H = 300 mm, C = 1200 − 0.4 × 300 = 1200 − 120 = 1080 mm. Minimum protective distance S = 528 + 1080 = 1608 mm, or about 1.61 m. The installed distance is 1.30 m, which is about 308 mm short. Therefore the protective field boundary is too close to the hazard for a person walking at 1.6 m/s. The cell does not meet the calculated requirement, and the near miss is consistent with an inadequate separation distance. The distance must be increased, or the total response/stopping time must be reduced, before production continues.
2. Evaluate the muting arrangement and the stop category. What assumptions are unsafe?
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Single-sensor muting is unsafe for personnel protection because one sensor can be triggered by a person, a stray reflector or a pallet in the wrong position. Muting should use at least two independent, diverse sensors arranged so that the sequence cannot be initiated by a person entering the field, and it should be limited to the pallet-exchange period with a maximum duration. The near miss during muting shows that the arrangement can allow entry while the hazard is active. A stop category 1 controlled stop also means the 250 ms figure must be the measured worst-case stop time with maximum load, maximum speed and worst-case brake or valve response, not a catalog value. If the actual stop time is longer, the protective distance increases further.
3. Propose corrective actions and a verification method before restarting the cell.
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Increase the protective field distance to at least 1.61 m, preferably with a margin, or reduce the scanner response, relay delay and robot stop time. Reconfigure muting with two independent sensors, a muting lamp, a maximum muting time and a layout that prevents a person from walking through the muted zone. Consider reducing robot speed in the approach zone or adding hard guarding at the pallet opening. Measure the actual robot stop time with a trace at maximum load and speed, and add that measured value to the calculation. Verify the final installation with a test piece moving at 1.6 m/s toward the hazard at several points along the field boundary; the robot must stop before the test piece reaches the hazard. Document the calculation, test results and muting logic.