Configuring Safety Laser Scanner Fields (Protective vs Warning) per ISO 13849 and ISO 13855
1. Distinguishing Protective Fields from Warning Fields
Optical Safety Laser Scanners (such as SICK MicroScan3, S3000, or Keyence SZ-V) utilize Time-of-Flight (ToF) infrared radar to scan a 275° radial plane, segmenting coverage into two zones: 1. Protective Field (Trip Field): Human-safety zone governed by ISO 13849-1 (PL d/PL e). Intrusions immediately de-energize the dual-channel OSSD (Output Signal Switching Device) outputs to 0V within milliseconds (< 60-80 ms), forcing category 0 or 1 E-Stops. 2. Warning Field (Outer Zone): Auxiliary non-safety zone that activates audible beepers or commands robotic arms and AGVs to ramp down velocity to 30% prior to entering hazard zones.
2. Minimum Safety Distance Formula per ISO 13855
Protective field contours cannot be set arbitrarily; compliance requires calculations under ISO 13855 (EN ISO 13855): $$S = (K \times T) + C$$ Where: - S: Minimum safety clearance between pinch hazard and protective field periphery (mm). - K: Human body approach velocity, standardized at 1,600 mm/s for horizontal entry. - T: Total aggregate system stopping latency ($$T = t_1 + t_2$$), where $$t_1$$ is scanner response plus safety relay transit time, and $$t_2$$ is mechanical machine stopping time. - C: Additional distance offset to account for physical limb reach or stride overrun (850 - 1,200 mm depending on optical elevation).
3. Practical Calculation Model: Industrial Robotic Workcell
| Parameter | Measured Metric | Engineering Basis |
|---|---|---|
| K (Approach Velocity) | 1,600 mm/s | ISO 13855 standard horizontal walking pace |
| t1 (Scanner + Safety PLC latency) | 0.085 s (85 ms) | Dual ToF evaluation scans + OSSD transistor de-energization |
| t2 (Robot Mechanical Brake Time) | 0.250 s (250 ms) | Measured worst-case physical deceleration to full standstill |
| T (Total Stopping Time T = t1 + t2) | 0.335 s | Total elapsed time from beam break to zero hazard velocity |
| C (Intrusion Margin) | 850 mm | Step-through intrusion offset for scanner mounted at 300 mm |
| S (Mandatory Clearance) | 1,386 mm | S = (1,600 * 0.335) + 850 = 536 + 850 = 1,386 mm |
4. Dual OSSD Wiring and Cross-Fault Detection
The scanner's OSSD 1 and OSSD 2 outputs continuously transmit brief test pulses (100-300 microseconds) while at 24VDC to verify short-circuit integrity. Safety Wiring Constraints: - Never bridge OSSD 1 and OSSD 2. - Always terminate into dual-channel fail-safe inputs on Safety Relays (e.g. SICK UE410, Pilz PNOZ) or Safety PLCs (Siemens F-CPU, GuardLogix).
Optical Window Contamination & Dust Compensation
Coolant mist or ambient dust accumulation on the scanner optics triggers an unrecoverable Optics Contamination Alarm. Countermeasures: Enable multi-sampling dust filtering in Safety Designer software and fit physical deflector shrouds to mitigate false plant stoppages.