Safety-Rated Monitored Stop (SRMS) in Collaborative Robotics

Definitional Boundaries & Taxonomy

Safety-Rated Monitored Stop (SRMS), increasingly referred to as "Monitored Standstill" in contemporary iterations of ISO 10218-1 and ISO/TS 15066, represents one of the four foundational paradigms of collaborative robot (cobot) operation. Unlike Power and Force Limiting (PFL), which permits physical human-robot contact, SRMS enforces strict spatial separation. The defining characteristic of SRMS is its non-destructive operational pause: when a human enters the collaborative workspace, the robotic system detects the intrusion via safety-rated sensors and executes a Category 2 stop (per IEC 60204-1). Crucially, the drive power to the actuators remains active. The robotic controller transitions into a "Safe Operating Stop" (SOS) state, actively monitoring the axes to ensure zero kinematic displacement. If the robot drifts beyond a micro-tolerance, the system triggers a Category 0 stop (Safe Torque Off - STO). This taxonomy clearly separates SRMS from an Emergency Stop (E-Stop); an E-Stop is a fault reaction requiring manual reset and heavy deceleration that drops the drives, whereas SRMS is a fluid, functional state allowing automatic resumption of the planned trajectory the moment the human clears the zone. It bridges the gap between high-speed industrial performance and human safety by leveraging real-time cyber-physical monitoring rather than rigid hardware disconnects.