A curated guide to collaborative robot safety (cobot safety), covering ISO 10218 requirements, ISO/TS 15066 specifications, Power and Force Limiting (PFL), safety architectures, and the latest in deterministic physics-based safety for modern cobots and humanoid robots.
Collaborative robots ("cobots") are designed to operate alongside human workers without traditional safety fences. Cobot safety is the set of standards, technologies, and practices that make this shared workspace safe.
ISO/TS 15066 defines four collaborative operation modes, all built on the foundation of ISO 10218:
- Safety-Rated Monitored Stop (SMS) — robot stops when human enters workspace
- Hand Guiding — human directly guides the robot via safety-rated control
- Speed and Separation Monitoring (SSM) — robot slows/stops as human approaches
- Power and Force Limiting (PFL) — contact forces/pressures limited so collisions don't cause injury
PFL is the most technically challenging mode — and also the most relevant for humanoid robots and VLA-controlled systems, where physical interaction is inherent to the task.
ISO 10218 is the binding international standard for industrial robot safety (ISO/TS 15066 is a technical specification that adds collaborative specifics). Key ISO 10218 principles:
- Safety functions must be deterministic and verifiable
- Risk assessment must be performed for each application
- Layered risk reduction is preferred over single safeguards
- Safety-rated software has specific development and validation requirements
PFL limits contact forces and pressures so that even if a collision occurs, it won't cause injury. ISO/TS 15066 Annex A defines force and pressure limits for different body regions under two conditions:
- Quasi-static — slow, sustained contact (clamping)
- Transient — quick, momentary contact (impact)
PFL compliance requires knowing both force and pressure. Pressure = Force ÷ Contact Area. But contact area isn't a constant — it changes dynamically based on:
- Material stiffness of both surfaces
- Impact force and velocity
- Geometry of the contact surfaces
- Body region (different tissues have different compliance)
Dynamic contact area calculation is essential for accurate PFL verification. Fixed-area assumptions lead to either unsafe conditions (area overestimated) or over-conservative operation (area underestimated).
A critical distinction for modern AI-powered robots:
- Deterministic safety — same input → same output, always. Based on physics, math, first principles. Auditable, verifiable. Required by ISO 10218 for safety-rated functions.
- Probabilistic safety — based on ML, statistics, training data. 99% accuracy means 1% failure rate. Not suitable as a primary safety mechanism.
For VLA-powered cobots and humanoid robots, the recommended architecture is: VLA handles planning and perception (probabilistic), while a deterministic physics safety layer provides the final safety gate.
Controller → Robot → Force Sensor → Threshold Check → Stop/Continue
Simple force thresholds, limited contextual awareness. Works for well-defined industrial tasks.
VLA Model → Action Plan → [Deterministic Safety Layer] → Controller → Robot
↓
over_ratio feedback
↓
VLA replanning (if needed)
The safety layer evaluates:
- Semantic plausibility — is this action physically possible?
- Dynamic contact area — what's the actual contact pressure?
- Impulse boundary — is the momentum within safe limits?
- Reaction force stability — will the robot remain stable?
Output is a 7-level risk assessment with over_ratio, enabling graduated responses rather than binary stop/go.
Rotor is a deterministic physics-based safety engine designed for collaborative robots, humanoid robots, and VLA-controlled systems. It provides the safety layer described above.
For cobot manufacturers
- Drop-in PFL verification aligned with ISO 10218 / ISO/TS 15066
- Dynamic contact area for more accurate (and less conservative) force/pressure limits
- Single file, zero dependencies — easy to certify
For VLA / humanoid teams
- Fills the "physical intuition gap" in VLA models
- Sub-millisecond latency — fits in real-time control loops
- 100% deterministic — safety behavior is predictable and auditable
- Works with any VLA model (Google ER 2, custom models, etc.)
For system integrators
- Works with any robot stack (ROS, custom controllers, etc.)
- JSON-based API — integrate from any language
- Comprehensive test suite — 349 tests, 100% pass rate
| Feature | Value |
|---|---|
| Latency | ~17μs per full check |
| Determinism | 100% — pure physics, no ML |
| Dependencies | 0 — single Python file |
| Action types | 35+ (grasp, push, pull, lift, etc.) |
| Risk levels | 7 (progressive, not binary) |
| ISO alignment | ISO 10218 + ISO/TS 15066 PFL |
| License | MIT (community) / Commercial (enterprise) |
| Standard | Type | Scope | Key Content |
|---|---|---|---|
| ISO 10218-1 | International Standard | Robot manufacturer | Robot design safety requirements |
| ISO 10218-2 | International Standard | System integrator | Integration and installation safety |
| ISO/TS 15066 | Technical Specification | Collaborative robots | SMS, hand guiding, SSM, PFL + Annex A body region limits |
| ISO 13849 | International Standard | Safety-related parts of control systems | PLd / PLe performance levels |
| IEC 61508 | International Standard | Functional safety | SIL (Safety Integrity Level) framework |
- Rotor Safety Engine — Open-source deterministic physics safety engine
- ISO 10218 Guide — Detailed ISO 10218 & ISO/TS 15066 reference
- Dynamic Contact Area — Deep dive into contact area physics
- Real-Time Robot Safety — Real-time safety architecture guide
- Humanoid Safety — Humanoid robot safety considerations
- Collision Detection — Physics-based collision detection
- Cobot safety
- Collaborative robot safety
- ISO 10218
- ISO/TS 15066
- Power and Force Limiting (PFL)
- Human-robot collaboration (HRC)
- Deterministic safety
- Physics-based safety
- Dynamic contact area
- Real-time safety
- VLA safety
- Humanoid robot safety
- Embodied AI safety
- Robot collision detection
MIT — educational and reference use. Always consult official standards for compliance.