Mitsubishi Electric's RV-FR series comes with SafePlus. Comau's Racer-5-0.80 carries SafeStop. Both are sold with the claim that you can remove the fence. Meanwhile, Universal Robots and FANUC CRX cobots are marketed as fence-free from birth. The marketing converges on the same four words — "no safety fence required" — but the safety engineering underneath could not be more different. One system is designed to prevent contact. The other is designed to survive contact safely. Confuse the two, and you are either wasting production uptime on unnecessary stops or exposing operators to risks the hardware was never designed to handle.
Collision avoidance vs. collision tolerance
An industrial robot with SafeStop or SafePlus relies on IEC 61800-5-2 safety sub-functions: Safe Torque Off (STO), Safe Limited Speed (SLS), Safe Limited Position (SLP). When a safety laser scanner or light curtain detects entry into a defined zone, the robot transitions to a monitored stop state — the drive power is cut via safety-rated relays, and the robot holds position. No contact is allowed to occur.
A cobot under ISO/TS 15066 relies on Power and Force Limiting (PFL). It is allowed to make contact — in fact, the entire safety case assumes contact will occur. The protection comes from the fact that joint torque sensors or dual-channel encoders detect the collision within 10 milliseconds and stop the robot before the transmitted force exceeds the body-region-specific limits defined in ISO/TS 15066: 150 N for the hand, 65 N for the face, and so on.
- SafeStop / SafePlus (preventive stop)
- "Do not enter the zone. If you do, I stop before I touch you."
- ISO/TS 15066 cobot (force-limited contact)
- "You can be in the zone. If I touch you, I stop before I hurt you."
The distinction matters because each approach has a set of scenarios where it is the right tool — and a set where it is dangerous or uneconomical.
Where SafeStop-class robots make more sense than a cobot
- High payload, high speed, low interaction. A 20 kg payload at 3 m/s TCP speed cannot be made force-limited — the kinetic energy is too high. But if the operator only enters the cell once per shift for inspection, stopping the robot for 90 seconds is perfectly acceptable. The stop cost is trivial; the payload/speed capability is non-negotiable.
- Certification path clarity. IEC 61800-5-2 safety sub-functions have a mature certification ecosystem. SIL 2 / PL d ratings are straightforward to achieve with off-the-shelf safety PLCs and sensors. ISO/TS 15066 cobot applications require application-level risk assessment that accounts for the specific end effector, workpiece geometry, and operator interaction pattern — a more involved and more expensive certification process.
- Mixed-mode operation. Many SafePlus-equipped robots can run at full speed behind a fence during production and switch to a safety-limited mode when the operator enters for inspection or maintenance. A cobot is always limited — you cannot temporarily disable force limiting when the operator leaves.
Where cobots are irreplaceable
- Continuous close-proximity work. Hand-guided assembly, where the operator physically moves the robot through a path, is impossible with a SafeStop robot — the constant stopping would destroy the workflow. A cobot's force-limited mode is the only option here.
- Shared workspace without zone switching. If the operator and robot share the same table surface simultaneously — picking from the same bin, passing parts back and forth — cobot force limiting is required. A safety scanner cannot practically distinguish between the operator reaching for a part and the robot approaching the same location.
- Rapid redeployment. Cobots are designed to be moved between stations without re-engineering the safety system. A SafeStop cell requires re-positioning and re-certifying safety scanners, recalculating stopping distances, and re-verifying PL ratings at each new location.
The blind spot that causes real accidents
The most dangerous configuration is neither a bare industrial robot nor an uncertified cobot — it is a SafeStop robot deployed as if it were a cobot. When operators are trained that the robot "stops when you get close," they begin to trust the stopping behavior and enter the zone more freely. Two failure modes follow:
- Scanner blind zones. A safety laser scanner scans in a single horizontal plane, typically 200–300 mm above the floor. An operator crouching to pick up a dropped part is below the plane. An operator reaching over the scanner to clear a jam is above it. In both cases, the robot does not stop — because the scanner never saw the intrusion.
- Stopping distance under load. A robot carrying a heavy payload at speed requires measurable stopping distance even with STO. If the safety zone boundary was calculated for an unloaded robot at reduced speed, a loaded robot at production speed may not stop before reaching the operator. The safety distance calculation must use the worst-case payload and the worst-case speed for the specific application, not the robot's minimum stopping distance from the datasheet.
Treat safety zone design as an engineering calculation with documented inputs, not a "point the scanner at the floor and hope" exercise. Every SafeStop deployment should have a stopping-distance measurement at full payload and full speed before production begins.
The cobot blind spot: the end effector is not certified
A UR10e or FANUC CRX-25ia carries a force-limited certification for the robot arm itself. The moment you bolt a pneumatic gripper, a welding torch, or a cutting blade to the flange, the safety case changes. ISO/TS 15066 force limits apply at the contact point — if a gripper with a 1 cm² sharp edge contacts an operator's hand at the same force the robot arm would exert over 20 cm², the pressure increases 20× and exceeds the allowable limit. The cobot arm cannot compensate for a dangerous end effector — the application-level risk assessment must account for the tool's contact area, edge radius, and temperature.
What is the actual stopping distance of a SafeStop robot under production load?
There is no single number, which is exactly the problem. A robot carrying 15 kg at 2.5 m/s TCP speed may require 400–800 mm of stopping distance including the safety-system reaction time (scanner detection + safety PLC processing + STO activation + mechanical braking). With 5 kg at 1 m/s, the same robot may stop in 150–250 mm. You must measure it for your worst-case payload-speed combination. Use a calibrated high-speed camera or a laser tracker to capture the actual stopping path during a triggered safety stop test. Never use the unloaded stopping distance from the robot datasheet as your safety zone boundary — it will be wrong by a factor of 2–4× under load.
Can I use a cobot for a 15 kg palletizing application without a fence?
Almost certainly not, and the reason is not the robot arm — it is the payload. A 15 kg box has significant inertia even at cobot-typical speeds of 1.0–1.5 m/s. If the cobot arm stops within 10 ms on collision detection, the payload continues moving forward — the robot stops, but the box does not. The ISO/TS 15066 force limits apply to the total effective moving mass, including the payload. A 15 kg payload plus the tool mass plus the robot link inertia will exceed allowable contact force for most body regions. This is why cobot palletizing cells typically still have light guarding — the cobot may be safe, but the payload it is carrying is not.
How do I tell if my end effector passes the cobot safety assessment?
Measure the contact area and edge radius of every surface that could contact an operator. For quasi-static contact (clamping, trapping), the allowable pressure under ISO/TS 15066 is 25–110 N/cm² depending on the body region. For transient contact (impact), it is 35–190 N/cm². If your gripper tip is a 0.5 cm² steel edge, even a 20 N collision force produces 40 N/cm² — exceeding the face limit. Two practical mitigations: (1) add rounded covers or compliant padding to increase contact area, or (2) reduce the cobot's maximum speed so that collision energy stays below the threshold. Both paths require documented force/pressure measurements as part of your application certification.
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