Robot cell safety basics: from hazard list to validated mode

Robot safety is a system property. A robot arm may have a certified controller, but the cell can still be unsafe because of a gripper, fixture, sharp workpiece, unexpected restart, access door, or maintenance procedure. Safety begins with the task and the people who can be exposed, not with the robot model.

Describe the work envelope

Map normal production, teaching, setup, recovery, cleaning, and maintenance. For each mode, identify motion, stored energy, pinch points, dropped loads, tool changes, sharp edges, and unexpected paths. Include the space behind the arm and the area a person can reach through or over a guard. A risk assessment that only watches the programmed path misses faults and recovery work.

Choose safeguarding that matches access frequency: fixed guards, interlocked doors, presence sensing, scanners, safe speed, enabling devices, or a combination. A collaborative application is not automatically safe because it uses a cobot; force, speed, tool, payload, posture, and contact location still require validation.

Make modes explicit

Automatic, manual, setup, and maintenance modes should have distinct permissions and safe speeds. A teach pendant enabling switch should not become a general bypass. Design restart behavior so closing a gate does not unexpectedly move the machine. Make the selected mode, safety state, and reason for a stop visible to the operator.

Safety functions need a defined response: stop category, monitored speed, safe position, torque removal, gripper release behavior, and reset conditions. The PLC or robot controller should not be the only place the safety case exists; keep the hazard, function, validation evidence, and change history together.

Commission and maintain

Test every safety device individually and in combinations that reflect real access. Check wiring, diagnostics, response time, restart prevention, and behavior after power restoration. Repeat tests after tooling, software, cell layout, or process changes. Record bypasses with an owner and expiry; a permanent bypass is an unapproved design change.

The ROS 2 industrial robotics guide explains the software boundary between planning and safety-rated control. The cobot glossary entry is a quick terminology reference. For the larger plant context, Understanding the Shop Floor is a practical ebook companion.