Safety laser scanner

Safety laser scanner – definition

A safety laser scanner is an active optoelectronic protective device that monitors a two-dimensional area around a machine or mobile robot. It emits laser pulses, measures reflected light by time of flight, and evaluates whether an object enters a configured protective field. When the field is infringed, the device switches its safety outputs to a defined safe state within its specified response time.

In mobile robotics, a safety laser scanner is used to reduce collision risk around an unmanned ground vehicle (UGV). It is not equivalent to a navigation LiDAR. Both devices may produce range measurements, but a safety laser scanner is designed, validated, and documented for safety-related protective functions. Applicable product requirements are defined by IEC 61496-3 for active optoelectronic protective devices responsive to diffuse reflection and by ISO 13849-1 for safety-related parts of control systems.

A scanner can protect the travel path, sides, loading zone, or service perimeter of a mobile platform. Its safety function normally acts through dual-channel safety outputs, often called OSSD outputs. These signals must be integrated into a compatible safety-related control system, such as a safety controller, safety relay, or certified motor-control architecture. A standard ROS 2 node is not a replacement for a safety-rated control path.

Safety scanner versus navigation LiDAR

A 2D LiDAR used for SLAM, obstacle avoidance, or mapping can publish high-quality range data, but this does not make it a safety component. Navigation software may fail because of operating-system delays, processor overload, stale transforms, incorrect parameters, communication loss, or algorithmic limitations. A safety scanner has internal diagnostics intended to detect specified faults and transition its safety outputs to the safe state.

Characteristic	Safety laser scanner	Navigation LiDAR
Primary purpose	Protective field monitoring	Perception, mapping, localization
Safety output	Safety-rated OSSD or safety fieldbus, model dependent	Usually Ethernet, USB, UART, or CAN data stream
ROS 2 role	Optional diagnostic or measurement interface	Common source of `sensor_msgs/msg/LaserScan`
Safety claim	Defined by manufacturer documentation and validated system design	No safety claim unless explicitly certified for that function

The ROS message sensor_msgs/msg/LaserScan represents planar range measurements with angular and timing metadata. It is useful for visualization and non-safety navigation, but the ROS interface must not be treated as the safety channel. The message definition is maintained in the ROS 2 sensor_msgs package.

Key parameters and protective-field design

Safety scanner selection requires more than choosing a maximum range. The safety function depends on the scanner specification, mounting position, braking behavior, control-system response, environmental conditions, and the geometry of the protected area. The final protective field must be calculated and validated for the complete UGV system.

Important parameters include the following:

Protective-field range – the maximum certified detection distance for the selected resolution and field configuration.
Detection capability or resolution – the minimum object size specified by the manufacturer for a protective function.
Response time – the time from field intrusion to a safe output transition.
Field switching time – relevant when different fields are selected for forward, reverse, turning, or stationary operation.
Scanning plane height – determines whether legs, pallets, low obstacles, or overhanging structures can be detected.
Stopping time – the measured time from removal of the motion command to complete platform stop.

The minimum separation distance is commonly derived from the approach speed and total stopping time. A simplified engineering form is:

S = K × T + C

In this expression, S is the required separation distance, K is an approach-speed parameter, T is the total response and stopping time, and C accounts for intrusion and detection effects. ISO 13855 defines the positioning principles for safeguards relative to approach speeds. The applicable calculation must use the current standard, the scanner manufacturer’s instructions, and measured vehicle stopping performance.

Integration on Leo Rover and Raph Rover

Leo Rover is a four-wheel differential-drive UGV. It runs ROS 2 on its onboard Raspberry Pi compute unit. A safety scanner can be mechanically mounted with its scanning plane positioned for the intended hazard zone. The scanner should not be connected only as a ROS 2 sensor if it is expected to perform a protective function.

For Leo Rover, a typical architecture separates safety and autonomy functions:

The scanner OSSD outputs connect to a safety relay, safety controller, or appropriately designed motor-enable circuit.
The safety circuit removes or inhibits traction when a protective field is breached.
ROS 2 receives non-safety status, diagnostic, or scan data when the scanner provides a separate communication interface.
The navigation stack publishes velocity commands, but it cannot override a safety stop.

Raph Rover can require a larger protective field because it is intended for higher payload applications. Its final field geometry must account for payload overhang, braking distance, turning envelope, sensor occlusion, and the location of the scanner relative to the outer vehicle contour. A field valid for a compact Leo Rover configuration cannot be transferred to Raph Rover without a new risk assessment and validation.

ROS 2 diagnostics and non-safety data

Where supported by the device manufacturer, ROS 2 can expose scanner measurements and diagnostics for visualization, logging, or supervisory behavior. This data path is useful for detecting contamination warnings, field-set selection state, or communication faults. It remains non-safety-related unless the entire path has a documented safety certification, which standard ROS 2 deployments do not provide.

ros2 topic echo /scan
ros2 topic echo /diagnostics
ros2 run tf2_ros tf2_echo base_link safety_scanner_link

A scanner frame should be represented in the robot TF tree using a fixed transform. The transform is relevant for visualization and perception, but changing it does not alter the physical protective field configured inside the safety scanner.

Standards and validation limits

A safety scanner does not make a UGV compliant by itself. Compliance depends on the complete safety function, including risk assessment, wiring, safety controller logic, drive behavior, emergency stop design, verification, and documentation. ISO 12100 describes machinery risk assessment and risk reduction principles. ISO 13849-1 specifies design principles for safety-related control systems, while ISO 13849-2 addresses validation.

For mobile platforms, field tests should include stopping-distance measurements at the maximum permitted speed, field-infringement tests across relevant approach directions, battery-voltage conditions, payload variants, surface conditions, and expected outdoor contamination. Rain, dust, fog, direct sunlight, reflective materials, and uneven terrain can affect optical sensing and must be assessed against the scanner manufacturer’s specified operating limits.

Sources: ISO 12100:2010; ISO 13849-1:2023; ISO 13849-2:2012; IEC 61496-3:2018; ISO 13855:2024; ROS 2 sensor_msgs/msg/LaserScan documentation.

Safety laser scanner