Perception and Sensors
Safety Laser Scanners: How Protective Fields Are Sized
The protective field length is a calculation, not a preference. Here is the formula, the constants that go into it, and the five inputs installers get wrong.
Field length is calculated, not chosen. The minimum distance is the approach speed multiplied by the total system stopping time, plus an intrusion allowance and a scanner-specific tolerance. With the conventional walking speed of 1,600 mm/s, every 100 ms of response time adds 160 mm before any machine stopping distance is counted.
The calculation
The minimum distance S combines four contributions:
- Approach. K multiplied by T, where K is the approach speed constant, conventionally 1,600 mm/s for walking, and T is the total response time of the whole chain.
- Machine stopping. The distance the hazardous motion travels after the stop command, which has to be measured, not estimated.
- Intrusion allowance C. Accounts for a limb entering the field before the body is detected. For a horizontal field with a defined resolution, this is a substantial fixed term.
- Scanner tolerance Z. Measurement error and, for horizontally mounted scanners, an additional allowance for reflective floors.
| Term | Value | Source |
|---|---|---|
| Scanner response time | 80 ms | Device datasheet, at the chosen resolution |
| Safety controller and network | 30 ms | Configuration dependent |
| Robot reaction to stop command | 40 ms | Robot manufacturer data |
| Total response time T | 150 ms | Sum of the above |
| Approach term, 1,600 x 0.150 | 240 mm | Calculated |
| Robot stopping distance | 320 mm | Measured at speed and payload |
| Intrusion allowance C | 850 mm | Standard term for horizontal detection |
| Scanner tolerance Z | 100 mm | Device dependent |
| Minimum distance S | 1,510 mm | Sum |
The five inputs installers get wrong
- Response time taken at the wrong resolution. Scanner response depends on the configured resolution and the number of scans required for a detection. A finer resolution is slower, and the datasheet has a table rather than a single figure.
- Stopping distance estimated rather than measured. It has to be measured at the actual programmed speed with the actual payload, and re-measured whenever either changes.
- Network latency omitted. Safety communication adds cycle time and a watchdog margin, and both belong in T.
- Reflective floor ignored. Polished or wet floors reflect the beam and can cause missed detection, which is exactly what the additional tolerance term exists for.
- Field not covering the whole approach. A person can step around a field, over it, or reach through a gap between a field and a wall. Gaps larger than the resolution are access routes.
Resolution and what it protects
| Resolution | Detects | Typical response | Use |
|---|---|---|---|
| 30 mm | Finger | slowest | Close protection at an opening |
| 40 mm | Hand | slow | Hand protection at a hazard point |
| 50 to 70 mm | Leg | medium | Floor field, approach detection |
| 150 to 200 mm | Body | fastest | Presence detection in a large area |
Finer resolution is not automatically better. It slows the response, which lengthens the required distance, and it increases nuisance trips from dust, chips and passing pallets. Choose the coarsest resolution that detects the body part which can actually reach the hazard.
Muting, blanking and the shortcuts that create findings
Real cells need material to pass where people must not, and the mechanisms that allow this are the most frequently misapplied part of a scanner installation.
| Mechanism | What it does | Legitimate when | Common misuse |
|---|---|---|---|
| Field set switching | Selects a different field by machine state | The switch is safety-rated and state-verified | Switched by the standard PLC alone |
| Muting | Suspends detection while material passes | Two independent sensors confirm material, with a timeout | A single sensor, or no timeout |
| Blanking | Ignores a fixed object inside the field | The object is genuinely fixed and monitored | Blanking a region a person can occupy |
| Reduced field on approach | Shrinks the field at lower speed | Speed is monitored by a safety function | Speed asserted by the program |
The pattern in the misuse column is identical in every case: a safety decision made by non-safety logic. If the condition that permits the field to change is not itself safety-rated, the protective function has been removed rather than adapted.
Frequently asked questions
How is a safety scanner protective field calculated?
Approach speed times total response time, plus the machine stopping distance, plus an intrusion allowance and a device tolerance. With the conventional 1,600 mm/s walking speed, each 100 ms of response adds 160 mm.
Why is my required distance over 1.5 metres?
Because the intrusion allowance for horizontal floor detection is a large fixed term, commonly 850 mm, which usually exceeds the approach and stopping terms combined. A faster machine stop does not remove it.
Does finer resolution improve safety?
Not automatically. Finer resolution slows the scanner response, which lengthens the required distance, and increases nuisance trips. Choose the coarsest resolution that detects the body part able to reach the hazard.
What must be re-measured after a change?
Machine stopping distance, whenever speed, payload or tooling changes, and total response time whenever the safety network or controller configuration changes. Both feed directly into the field calculation.
Can a person step around the field?
Yes, and that is a common finding. Any gap larger than the detection resolution between the field and a physical barrier is an access route, and the assessment must show that every approach path is covered.
Sources
- ISO 13855, positioning of safeguards with respect to approach speeds of parts of the human bodyInternational Organization for Standardization, the approach speed constants and intrusion allowances
- ISO 10218-1:2025, Robotics, Safety requirements, Part 1International Organization for Standardization, protective device requirements
- ISO 12100, safety of machinery, general principles for designThe risk assessment framework in which the calculation sits