Safety-Rated Monitored Stop

The function is activated within a predefined spatial zone, often monitored by safety-rated sensors like light curtains, safety laser scanners, or area scanners. This zone is distinct from the robot's full operational envelope and is configured during the system's safety validation. The robot's motion is permitted only when this zone is confirmed to be unoccupied by a human.

It implements a Category 2 stop per IEC 60204-1 and ISO 13850. This means:

• Motion is halted using a controlled stop (deceleration to zero).
• The robot's actuators remain powered on.
• The robot maintains its position and does not enter a zero-torque or gravity-compensation mode.
• All control system state information (joint positions, program counter) is preserved, enabling automatic resumption.

This is the defining characteristic versus a standard protective stop. Once the collaborative workspace is verified as clear by the safety sensor system, the robot automatically resumes its programmed task from the exact point of interruption. No manual restart command (e.g., pressing a reset button) is required, which is critical for maintaining workflow fluidity in collaborative applications.

The entire control loop—from sensor input to motion halt—must be implemented with Safety Integrity Level (SIL) or Performance Level (PL) rated components and architectures (e.g., PL d per ISO 13849-1). This includes:

• Dual-channel monitoring of stop signals.
• Cross-checking of sensor data and actuator feedback.
• Diagnostic coverage to detect faults within the safety function itself. The system must be validated to ensure the stop is always triggered within the required safety distance.

This mode is designed for tasks where human intervention is frequent but brief. Common use cases include:

• Machine tending: A human enters to load/unload a part; the robot stops, then resumes the machining cycle.
• Assembly assist: A robot holds a component; a human enters to perform a manual operation (e.g., inserting a pin), then leaves for the robot to proceed.
• Quality inspection: A robot presents a workpiece; a human enters for a visual check before the robot moves it to the next station.

It is one of four primary collaborative operations per ISO/TS 15066:

• Safety-rated monitored stop: Stops on entry, auto-resumes on exit (this function).
• Hand guiding: Human physically directs robot motion.
• Speed and separation monitoring: Robot speed dynamically scales based on human proximity.
• Power and force limiting (PFL): Robot is inherently safe for contact via design limits. Unlike PFL, monitored stop prevents contact entirely. Unlike speed and separation, it uses a binary occupied/unoccupied state.

Hand Guiding is a collaborative operation mode where a human operator physically grasps the robot's end-effector or arm and manually moves it. The robot's control system enters a zero-gravity or compliant state, allowing effortless movement for teaching or direct task execution.

• Primary Use Case: Kinesthetic teaching of paths, points, or complex trajectories without traditional programming.
• Safety Integration: Often combined with a dead-man switch or enabling device the operator must hold. The system typically monitors for excessive speed or force.
• Control Scheme: Implemented via admittance control, where an applied force is interpreted as a desired velocity or position change.

A Collaborative Robot (Cobot) is a robot designed from the ground up to operate safely alongside humans in a shared workspace without traditional fixed safety guarding. Safety-rated monitored stop is one of its foundational capabilities.

• Inherent Safety Features: Include rounded edges, force-limited joints, smooth surfaces, and back-drivable actuators to minimize injury risk during contact.
• Enabling Technologies: Integrate collision detection (via proprioceptive sensing) and external safety-rated vision systems to enable modes like PFL and SSM.
• Application Scope: Distinct from traditional industrial robots retrofitted with sensors for collaboration; cobots are intrinsically safe for direct HRI.

Adjustable Autonomy is a system design principle that allows for dynamic, context-sensitive modification of a robot's level of self-governance. Safety functions like monitored stop are often triggers or components within these adaptive control frameworks.

• Operational Spectrum: Enables smooth transitions between fully autonomous, semi-autonomous (shared control), and fully manual modes.
• Integration with Safety: A monitored stop may be invoked to freeze the scene before control authority is transferred from the robot to a human operator, or vice-versa, ensuring safe handover.
• Use Case: In a complex assembly task, the robot may work autonomously, enter a monitored stop when the human brings a new part, then switch to hand guiding mode for a precise placement operation before resuming autonomous fastening.