How Collaborative Robots Avoid Injuring People

Industrial robots are normally kept inside a cage — they move fast and hit hard, making human contact dangerous. Collaborative robots (cobots) operate without that cage, right next to people. That is only acceptable if "contact won't injure a person" is guaranteed. ISO/TS 15066 defines that guarantee.

The Core: Contact Forces — Two Types of Contact

The central idea in 15066 is PFL (Power and Force Limiting) — constraining the force a robot can generate to stay within the limits the human body can tolerate. There are two types of contact.

Rendering diagram…

• Transient contact — a brief impact followed by immediate rebound. Because it is short, a relatively higher force is tolerated.
• Quasi-static contact — pressing a person against a fixed surface with sustained force. Because it persists, the limit is much lower.

These limits also vary by body region — the back of the hand can take considerably more than the face or neck. ISO/TS 15066 specifies force and pressure thresholds by body part.

💡 Same robot, different rules depending on where it hits — a gentle push on the hand may be fine; the same force applied to the neck is dangerous. Collaborative tasks must therefore constrain robot speed and force based on which body regions the robot could reach.

The Other Approach — Speed and Separation Monitoring (SSM)

Beyond limiting force, another strategy is to prevent contact from happening at all — SSM (Speed and Separation Monitoring). The robot continuously monitors its distance from the human: it slows down as the person approaches and stops if they get too close.

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This is the same principle as AMR safety-distance maintenance, applied to robot arms.

The Role of Simulation

▶ A collaborative workcell — the arm works inside a fence, a person (mannequin) stands right beside it, and the status light tower is lit at the top right. Contact force is not visible in this frame — force is measured by instrumentation and enters the verdict; the clip only shows the arrangement in which that force arises.

⚠️ Verifying contact forces on hardware alone is a nightmare — testing every combination of body part, posture, and velocity with a physical dummy is an unbounded problem. Computing contact forces in simulation first narrows the dangerous regions, leaving only the critical cases for physical verification. This approach grew more important when the 2025 ISO 10218 revision absorbed 15066's requirements into the main standard.

ISO/TS 15066 was published in 2016 as a Technical Specification. In the 2025 ISO 10218 revision, its safety requirements were folded into the main standard — making collaborative robot safety a core requirement rather than an annexe.

In a nutshell

📌 ISO/TS 15066 requires cobots to stay within human pain-threshold forces on contact (PFL) — with separate limits for transient and quasi-static contact, differentiated by body region. Alternatively, SSM enforces deceleration and stop based on separation distance. Verifying every combination on hardware is impractical, making simulation-based contact force analysis essential; the 2025 ISO 10218 revision integrated these requirements into the main standard.