Robot cell: light curtain vs scanner
Perimeter vs area protection. Use this engineering guide to choose between safety light curtains (perimeter/opening guarding) and safety laser scanners (area/zone guarding with Speed & Separation Monitoring). Includes design rules, distance math inputs, and acceptance records for audits.
Non-negotiable: The safety stop path is Type 4 / PL e with dual OSSD channels → safety relay/PLC → force-guided contactors K1/K2. PLC logic only supervises and interlocks; it does not perform the safety stop.
1) Strengths at a glance
Light curtain — perimeter/opening
- Best for access points: doorways, material in/out chutes, teach gates.
- Fast response; small safety distance using
S = K × T + Cwith 10/30/40 mm resolutions. - Simple diagnostics, low cost per opening; easy EDM wiring and reset control.
Safety laser scanner — area/zone
- Creates 2D protective fields (up to 270°) for whole-side guarding or SSM*.
- Multiple field sets for mode switching (auto/teach/maintenance).
- Excellent for tight cells where fences aren’t feasible; handles curved paths/AGV aisles.
*SSM = Speed & Separation Monitoring: slow/stop robot when a person approaches.
2) Quick decision table
| Design question | Prefer light curtain | Prefer laser scanner |
|---|---|---|
| Guarding type | Perimeter at fixed openings | Area coverage; irregular edges |
| Human approach | Single direction, predictable | Multi-direction, unpredictable |
| Safe distance pressure | Short distance needed; fast T | Distance set by field geometry |
| Muting/material flow | Simple two-sensor muting OK | Dynamic zones following material or AGV |
| Budget per entry | Lower per opening | One scanner may replace multiple curtains |
| Dust/weld spatter | Use IP65/67/69K + window covers | Use filters/heaters; keep lens clear |
3) Proven patterns for robot cells
A. Perimeter fence + light curtains
- Fences on three sides; curtains at entries with edge-reset + EDM.
- Interlock doors (safety switches) tied into the same stop chain.
- Use blanking/muting only for necessary material pass-through with lamp indication.
B. Full scanner guarding (no fence)
- One or two scanners creating protective fields across open sides.
- Switch to wider warning fields for approach slowdown (SSM).
- Place scanners at heights/angles that block crawl-under & reach-over.
C. Hybrid: scanner + light curtain
- Scanner covers long faces; curtain protects narrow load ports.
- Shared EDM feedback from K1/K2; unified reset policy.
- Field set changes when doors open / robot in teach mode.
4) Safety distance & timing inputs
Light curtain worksheet
Resolution (mm): 10 / 30 / 40
Response times (ms): Curtain=?, Safety relay=?, K1=?, K2=? → T_total = sum
Approach speed K (mm/ms): hands/arms standard
Additional reach C (mm): per tooling/standards
Safety distance: S = K × T_total + C
Mounting: protective height OK; prevent reach-under/overScanner field design notes
- Set Protective Field to stop; Warning Field for SSM slowdown.
- Cover dead zones with mirrors/second scanner; verify reflectivity.
- Validate switching inputs (modes) and teach-in seals/records.
5) Wiring baseline (both options)
// Dual OSSD to safety controller; EDM feedback with K1/K2 NC in series
OSSD1 ─────► IN_CH_A K1 NC ─┐
OSSD2 ─────► IN_CH_B ├─► EDM/FB
Reset PB ─► RST/Y1-Y2 K2 NC ─┘
Safety OUTs 13-14 / 23-24 ─► Contactors K1/K2 (power removal)
Aux/Diag 41-42 (optional to PLC; not safety)Replace with your certified function blocks. Shown logic is supervisory only.
6) Commissioning & periodic tests
Commissioning
- Verify dual-channel coherence; EDM blocks reset with held K1/K2.
- Capture distance math, field screenshots, wiring photos.
- For scanners: verify field switching, blind-spot coverage, lamp/status IO.
Quarterly tests
- Light curtain test rods 10/30/40 mm; measure real drop times.
- Scanner: approach tests from all sides; SSM slowdown then stop.
- Seal parameters; review change logs.
Test log (CSV copy)
Item,Test,Expected,Actual,Result,Notes
1,EDM welded-contact block,Reset inhibited when K1/K2 held,,,
2,Curtain distance,S = K×T + C verified,,,
3,Scanner protective field,Stop on entry from all sides,,,
4,SSM warning field,Slowdown then stop within limits,,,
5,Mode switching,Correct fields active per mode,,,
7) Common mistakes & quick fixes
| Mistake | Symptom | Fix |
|---|---|---|
| Using PLC for safety stop | Unexpected restart | Use safety relay/PLC + K1/K2; PLC only monitors |
| Scanner blind spots | People slip past fields | Re-aim, add mirrors/second scanner; raise field height |
| No muting indication | Operators unaware | Dedicated muting lamp linked to logic |
| Single-channel OSSD | Hidden single-fault | Always dual channel with independent harness/fusing |