A safety-rated monitored stop is a collaborative operating mode in robot safety in which an industrial robot comes to a standstill before a person enters the shared workspace, and a safety-rated control function continuously verifies that it stays motionless while the person is present. When the person leaves, the robot resumes its program automatically without a manual restart.
In robotics standards, the concept appears in ISO 10218 as one of the four classic methods of collaborative operation, alongside hand guiding, speed and separation monitoring, and power and force limiting. It is the simplest of the four, because the robot never moves while a human is inside the collaborative zone; its value lies in removing the reset step that an ordinary guard opening would require.
How a safety-rated monitored stop works
The function relies on two elements: a presence detection device that recognizes a person approaching or entering the collaborative workspace, and a safety controller that commands the stop and supervises standstill. Typical detection devices are safety laser scanners, light curtains, safety mats or a manual request button pressed by the operator.
When presence is detected, the robot decelerates along its path and stops. Power to the drives remains on, which corresponds to a stop category 2 in the sense of IEC 60204-1. The safety controller then monitors joint positions with safety-rated encoders. If any axis moves beyond a small tolerance, the system triggers a protective stop that removes drive power.
The sequence below shows the states that a correctly configured cell passes through.
| Step | System state | Safety function active |
|---|---|---|
| Person approaches | Robot running at programmed speed | Presence detection armed |
| Detection triggered | Robot decelerates and stops on path | Controlled stop command |
| Person in shared zone | Drives powered, robot motionless | Standstill monitoring |
| Unexpected motion | Drive power removed | Protective stop, category 0 or 1 |
| Person leaves | Program resumes automatically | Monitoring released |
Requirements and key parameters
The stop and standstill functions must be safety-rated, which means they are implemented in control hardware and software designed and validated to functional safety standards. ISO 10218 generally expects robot safety functions to reach performance level d with category 3 architecture under ISO 13849-1, or an equivalent safety integrity level, unless the risk assessment justifies a different level.
The most important design parameter is the stopping distance. The robot must be fully stopped before the person can reach it, so detection devices are placed according to the separation distance formulas of ISO 13855, which combine human approach speed, system response time and the robot’s stopping time.
| Parameter | Typical source or range | What it affects |
|---|---|---|
| Human approach speed | 1,600 or 2,000 mm/s in ISO 13855 | Required detection distance |
| Robot stopping time | Tens to hundreds of milliseconds | Distance the robot travels after the stop command |
| Detection response time | Device datasheet, often below 100 ms | Total reaction delay |
| Standstill tolerance | Small angle window per axis | Sensitivity of motion supervision |
| Performance level | Typically PL d, category 3 | Reliability of the safety function |
Stopping time depends on payload, speed and arm extension, so it must be measured or taken from the manufacturer’s data for the worst case, not the average case.
Comparison with other collaborative operation methods
A safety-rated monitored stop differs from the other collaborative methods in one decisive way: robot and human never move together in the shared space. This makes it easy to validate but limits productivity whenever people enter frequently.
| Method | Robot motion with person present | Typical use |
|---|---|---|
| Safety-rated monitored stop | None, standstill is monitored | Manual loading of a fixture |
| Hand guiding | Only under direct operator control | Teaching positions, assisted lifting |
| Speed and separation monitoring | Yes, slowed with distance | Shared aisles and mobile manipulators |
| Power and force limiting | Yes, contact forces restricted | Close assembly beside workers |
| Emergency stop | None, but restart needs a manual reset | Abnormal situations, not routine work |
The comparison with the emergency stop is a frequent source of confusion. An emergency stop is a complementary protective measure for hazardous events, it removes power and it requires deliberate reset. A monitored stop is a routine operating state designed for planned human entry.
When to choose a monitored stop
The method suits tasks in which a person briefly works inside the robot’s reach at predictable moments, such as loading parts, inspecting a workpiece or removing finished assemblies. It also allows conventional high-payload robots to be used in collaborative applications, because the robot is stationary whenever contact could occur.
| Situation | Suitability | Reason |
|---|---|---|
| Operator loads a fixture every few minutes | Well suited | Short, planned entries with automatic restart |
| Worker and robot assemble side by side continuously | Poorly suited | Robot would stand still most of the time |
| Heavy payload or sharp tooling | Suited with care | Contact is avoided, but stopping distance grows |
| Mobile robot sharing open floor space | Usually unsuitable | Speed and separation monitoring fits better |
Common mistakes and limitations
Most failures of monitored stop designs come from incorrect assumptions about timing and coverage rather than from the safety controller itself.
- Underestimated stopping distance: measuring at low speed or light payload leaves the detection zone too small for real operation.
- Detection gaps: a person can reach in over or around a light curtain, so reach-over and reach-around must be covered in the risk assessment.
- Hazards that persist at standstill: a stopped robot can still hold a hot tool, a sharp edge or a part that may drop, so the tooling needs its own protective measures.
- Treating it as lockout: a monitored stop is not suitable for maintenance; lockout/tagout procedures still apply when energy must be isolated.
In simple terms
A safety-rated monitored stop is a pause button that a person triggers simply by walking in. A sensor notices the person, and the robot freezes in place. A safety system then watches the robot closely to make sure it really stays still.
When the person steps back out, the robot picks up exactly where it left off. Nobody needs to press a reset button. It is similar to an automatic door that stays open while someone stands in the doorway. The robot never moves while a person is within reach, which keeps the arrangement simple and safe.
A practical example
A metalworking shop uses a medium-payload robot to tend a press brake. Every few minutes an operator places a stack of raw blanks on a table inside the robot’s reach. A safety laser scanner monitors the floor in front of the table, and the robot’s safety controller is configured for a monitored stop.
During commissioning, the integrator measures the worst-case stopping time with the full gripper load at maximum speed and sets the scanner field so the robot is fully stopped before the operator can reach the table. When the operator steps in, the robot halts with drives powered. When the operator leaves, it continues within a second. The shop avoids a full fence around the loading table and the manual reset after every entry, while the operator never shares space with a moving robot.