Robot Safety Systems: Safeguarding Devices and Collaborative Operation

Robot Safety Systems: Safeguarding Devices and Collaborative Operation

Industrial robots are powerful, fast, and unpredictable to anyone who does not understand their programming. Effective safeguarding keeps workers safe by separating people from hazardous motion or, in collaborative applications, by controlling how the robot moves when a person is nearby. The governing standard in the United States is ANSI/RIA R15.06 (aligned with ISO 10218), which sets minimum requirements for the design, installation, and use of industrial robot systems.

Why Safeguarding Matters

A robot executing a looping program does not know a technician has stepped into its work envelope. Without safeguarding, a collision can cause crushing, striking, or entanglement injuries. OSHA 1910.212 requires machine guarding for all machinery, and ANSI/RIA R15.06 applies that requirement specifically to robot cells. Before any safeguarding work, hazardous energy must be controlled under OSHA 29 CFR 1910.147 (lockout/tagout). Electrical work near live circuits requires compliance with NFPA 70E, including verification that voltage is absent before contact. Industrial machinery wiring must meet NFPA 79.

Types of Safeguarding Devices

Hard guarding (perimeter fencing) creates a physical barrier. Fences must be tall enough and close-meshed enough that a person cannot reach over, under, or through them into the hazard zone. R15.06 provides guidance on fence height and opening size relative to the distance from the hazard.

Safety gate interlocks control access points in the fence. When a gate opens, an interlock switch signals the robot controller to stop. Interlocks are typically dual-channel (two separate circuits) so that a single wiring fault does not defeat the safety function. The gate must not be bypassable without a tool.

Light curtains project an infrared beam array across an opening. Breaking any beam triggers an immediate stop command. The curtain must be mounted far enough from the hazard that the robot stops before a hand can reach the danger zone — this distance is called the minimum safety distance.

Emergency stops (E-stops) are manual, non-automatic devices that, when activated, remove power or initiate a controlled stop. On a FANUC system, E-stops appear on the teach pendant, the controller front panel, and optionally on external operator stations. R15.06 requires E-stops to be readily accessible, colored red on a yellow background, and self-latching so they cannot restart automatically.

Safety mats and bumpers detect presence on the floor or contact on the robot body and initiate a stop. These are common in collaborative zones.

Stop Categories

IEC 60204-1 (referenced by NFPA 79 and ANSI/RIA R15.06) defines three stop categories:

• Category 0 – Power is removed immediately. The robot coasts to a stop. This is the fastest removal of energy but the least controlled — the robot may drift from its path.
• Category 1 – The robot decelerates under power to a controlled stop, then power is removed.
• Category 2 – The robot decelerates under power to a controlled stop, and power remains applied to hold position.

Light curtains and safety gates most often trigger Category 0 or Category 1 stops depending on system configuration. Knowing which category a device triggers — and verifying it matches the design spec — is part of the functional test you will perform today.

Collaborative Robot Operation: Four Modes

ANSI/RIA R15.06 and ISO 10218-2 define four modes that allow a robot to operate in the same space as a person under specific conditions. Traditional safeguarding (full stop on entry) is still valid; these modes are alternatives when the task requires human presence.

1. Safety-Rated Monitored Stop (SRMS): The robot holds position, with drives still energized, while a person is in the collaborative workspace. The robot may resume only after the person has left. This is the most conservative collaborative mode.

2. Hand Guiding: An operator physically guides the robot by hand using a device mounted near the end-effector. The device includes an enabling switch and force sensor. Speed is limited, and the operator must hold the enabling switch to allow motion.

3. Speed and Separation Monitoring (SSM): The robot monitors the distance between itself and a detected person (using a safety laser scanner, vision system, or similar sensor). As the person approaches, the robot slows; if the person enters a defined protective zone, the robot stops. When the person moves away, the robot can resume without a manual reset. This is one of the most common collaborative modes in flexible manufacturing.

4. Power and Force Limiting (PFL): The robot limits the mechanical power and force it can deliver so that a collision causes no injury exceeding thresholds defined in ISO/TS 15066. Collaborative robots (cobots) designed under this mode use compliant joints, torque sensing, or current monitoring to detect contact and stop instantly.

Minimum Safety Distance Concept

For light curtains and similar presence-sensing devices, R15.06 references the formula from OSHA and ANSI B11 standards:

Ds = K × (Ts + Tc + Tr + Tbm)

Where:

• Ds = minimum safety distance (inches or mm)
• K = hand speed constant (typically 63 in/s per OSHA guidance)
• Ts = stopping time of the machine
• Tc = response time of the control system
• Tr = response time of the safeguarding device
• Tbm = additional time for brake monitor, if used

The result tells you how far the light curtain must be from the hazard so the machine stops before a hand can travel from the curtain to the danger point.

Safe Work Practices During This Lesson

Before triggering any safeguarding device, confirm with your instructor that the robot is in its approved looping idle program and that no one else is inside the cell. Wear safety glasses at all times in the lab. Never defeat or bypass a safety device — even for testing — without written authorization and a documented safe state. Log every test result immediately; do not reconstruct the log from memory after the session.