When to use: Compute the minimum protective separation distance for a speed and separation monitoring (SSM) collaborative or safeguarded robot cell per ISO/TS 15066 Annex A and ISO 13855 — the distance a safety-rated sensing zone must maintain so a person can never reach the hazard before the robot has safely stopped.
This calculator computes the minimum protective separation distance (S) for a Speed and Separation Monitoring (SSM) collaborative robot application, per the formula in ISO/TS 15066 Annex A and ISO 13855:2010. SSM is one of four collaborative operation types defined in ISO/TS 15066 — the robot and person can share a workspace, but the safety system must maintain enough separation that a person can never reach the robot before it has come to a complete, verified stop.
S = K × (Ts + Tr) + C + Zd + Zr. Each term represents a distinct source of delay or uncertainty that eats into the available reaction distance: K is a standardized human approach speed constant (1600 mm/s per ISO 13855, representing a fast but not sprinting approach). Ts is the robot system's stopping time — how long it takes the robot to actually come to a complete stop once commanded. Tr is the sensor and control system's response time — the delay between a person entering the detection zone and the stop signal actually being issued. C is the intrusion distance, which accounts for how far a person (or part of a person, like an arm) can move into the sensing field before the sensor registers the intrusion — it depends directly on the sensor's detection capability/resolution. Zd and Zr are position uncertainties for the safeguarding device and the robot respectively, covering things like sensor mounting tolerance and the robot's own stopping-position repeatability.
Ts (robot stopping time) is measured or specified by the robot manufacturer for a category 0 or category 1 stop at the relevant speed and payload — modern collaborative robots typically publish this, and it is also directly measurable with the actual application program and payload since stopping distance/time varies with speed, payload, and joint configuration. Tr (response time) comes from the safety sensor and safety controller/PLC datasheets — a safety-rated area scanner, light curtain, or safety mat all publish a response time, and any safety relay or safety PLC in the signal path adds its own processing delay on top. C (intrusion distance) is defined by ISO 13855 based on the sensor's detection capability (its ability to detect an object of a given size) — a value of roughly 850mm is a commonly-cited reference figure for typical area-scanner resolution, but the actual figure must come from the specific sensor's ISO 13855 table lookup or manufacturer guidance, since finer-resolution sensors (able to detect a finger vs. a whole body) reduce C substantially.
ISO/TS 15066 defines four distinct collaborative operation types, and SSM (which this calculator supports) is only one: (1) Safety-rated Monitored Stop — the robot fully stops whenever a person is in the collaborative workspace, and can only resume once they leave; (2) Hand Guiding — the robot only moves while an operator actively drives it via a hand-held device, at reduced speed; (3) Speed and Separation Monitoring (SSM) — the robot dynamically adjusts its speed (or stops) based on real-time measured separation distance to a person, using this minimum-distance formula as the trigger threshold; (4) Power and Force Limiting (PFL) — the robot itself is inherently limited (by design or software) to forces and pressures below the biomechanical limits in ISO/TS 15066 Annex A, allowing direct, intentional contact without a separate separation-monitoring system. Many commercial "cobots" are PFL-rated for direct contact at low speed but still use SSM logic (often built into the vendor's safety software) when moving at higher speed for productivity, switching modes based on measured distance to a person.
Enter the robot's stopping time and the safety sensor's response time from their respective datasheets or measured test data — these usually dominate the result. Enter the intrusion distance appropriate for your specific safety sensor's detection capability (consult ISO 13855's detection-capability table, or the sensor manufacturer's stated value), and reasonable position-uncertainty allowances for both the sensor mounting and the robot's own stopping repeatability. The result, S, is the minimum distance the safety-rated sensing field's boundary must be positioned from the nearest point the robot (or its payload/tooling) can reach — this is a minimum, not a target; real installations commonly add margin, and the final value should be validated by whoever performs the formal risk assessment for the cell, not taken directly from a calculator.
No. This tool computes the standard ISO/TS 15066 Annex A formula from inputs you provide, which is a necessary part of specifying an SSM safeguard, but a compliant risk assessment (per ISO 12100 and ISO 10218-2 / ISO/TS 15066 for the specific application) involves hazard identification, risk estimation, and validation testing that goes well beyond this one distance calculation. Treat the output as an engineering estimate to inform your design, not a certified safety distance — final sign-off should come from a qualified safety engineer for the specific installation.
ISO 13855's 1600 mm/s figure is deliberately conservative — it represents a fast approach (closer to a brisk walk/light jog) rather than typical relaxed walking speed (roughly 1200 mm/s), because the safety calculation has to protect against a worst-case realistic approach, not an average one. Some older versions of related standards used different constants (e.g. 2000 mm/s in some contexts), so always confirm which figure your applicable standard version specifies rather than assuming 1600 mm/s universally applies.
Tr is the delay before the stop command is even issued — the time for the safety sensor to detect the intrusion and the safety controller to process that signal and command a stop. Ts is the delay after that command is issued before the robot has actually, physically come to a complete stop — determined by the robot's deceleration capability at its current speed and payload. Both delays add directly to how far a person can travel before the robot is actually stopped, which is why the formula sums them together as (Ts + Tr) before multiplying by approach speed.
Not necessarily in the way you might expect — a slower-moving robot generally has a shorter stopping time (Ts), which does reduce the required distance, but the human approach speed term (K) does not depend on the robot's speed at all, since it models how fast a person can approach, independent of what the robot is doing. Reducing robot speed helps mainly by reducing Ts (and, in power/force-limited applications, by reducing the biomechanical severity of any contact) — it does not eliminate the need for adequate separation distance in an SSM system.
C is directly tied to a sensor's detection capability per ISO 13855 — a sensor that can only reliably detect a large object (like a whole leg or torso) has a coarser resolution and therefore a larger C, because a smaller part (a hand or arm) could intrude further before being detected. A finer-resolution sensor (able to reliably detect a small object like a finger) has a much smaller C. This is why upgrading to a higher-resolution safety sensor is a common way to shrink the required minimum protective distance without changing the robot's speed or stopping performance at all.
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