TL;DR:
A collaborative robot is designed to work in shared space, but safety comes from the whole system, tool and task included, not from the arm alone.
Speed and separation monitoring slows or stops the robot based on how far away a person is, using a distance calculated from speeds and stopping time.
Force limits and a written risk assessment cover the contact that cannot be avoided.
Traditional industrial robots work inside cages. A collaborative robot, or cobot, is meant to work in the same space as a person: handing over parts, holding a workpiece, or sharing a bench. Taking away the cage does not take away the danger. It moves the job of staying safe into the robot's design and its settings.
What collaborative means
Safety standards for industrial robots and their collaborative use (ISO 10218 and the technical specification ISO/TS 15066) describe four ways to work together: a safety-rated stop when a person enters, hand guiding, speed and separation monitoring, and power and force limiting. A real installation uses one or more, chosen by the task.
A cobot is not automatically safe. A sharp tool, a heavy payload or a fast move can hurt someone even on a robot built for sharing space. The standards apply to the complete application.
Worked example: a robot that slows when someone walks close
This is a simplified version of speed and separation monitoring, for learning. A sensor tracks where the person is. The robot needs a protective separation distance S: the minimum gap at which it must already be stopped or safe. It is built from three parts.
How far the person moves while the robot reacts and stops. Take a walking speed of 1.6 m/s, a figure commonly used in safety calculations. With a sensor and controller reaction time of 0.1 s and a robot stopping time of 0.3 s, the person moves
1.6 × (0.1 + 0.3) = 0.64 m.How far the robot moves before it stops. At 0.5 m/s: during the 0.1 s reaction it covers
0.5 × 0.1 = 0.05 m. While braking over 0.3 s at constant deceleration it covers about half of0.5 × 0.3, which is0.075 m. Total:0.125 m.A margin
Cfor sensor error and reaching, say 0.2 m for this example. A person's arm can also extend past their body, so real calculations include an intrusion allowance.
Add them: S = 0.64 + 0.125 + 0.2 = 0.965 m, so call it 1.0 m. If the person is farther than 1.0 m, the robot may work at 0.5 m/s. If they come within it, the robot must already be stopped or slow enough that the numbers still work.
Check the effect of slowing down. If the robot drops to 0.2 m/s inside a warning zone, its own travel falls to 0.2 × 0.1 + 0.5 × 0.2 × 0.3 = 0.02 + 0.03 = 0.05 m (assuming the same stopping time), and the separation becomes 0.64 + 0.05 + 0.2 = 0.89 m. The person's own walking distance, 0.64 m, dominates, which is why real systems also shorten reaction time with fast sensors and fast stopping.
A real design uses the formulas and values in the standard, measured stopping times, and a safety-rated controller. The numbers here show the shape of the calculation, not a design to copy.
Force limits
Sometimes contact is expected, as when a person guides the arm or leans into the workspace. Power and force limiting keeps the force and pressure of a contact below limits that depend on the body region. The standard gives tables, and the limits are lower for the head and face than for the hand or the large muscles of the arm. The robot senses unexpected force through motor current or joint sensors and stops.
What matters physically is pressure and the type of contact. A blunt rounded surface spreads the force. A sharp tool or a pinch against a fixed object concentrates it, so a gentle robot with a pointed gripper can still injure.
Risk assessment
A risk assessment is a written process: list each hazard (pinch points, tool edges, falling payloads, unexpected starts), estimate how severe an injury could be and how likely, then reduce the risk. The preferred order is to design the hazard out, then add guards or safety devices, then add warnings and training. Then you check the result and repeat when the task changes.
Not a design guide: This article explains ideas. Do not rely on its numbers to set up a real robot cell. Real installations need the current standards, a documented risk assessment, safety-rated components, and people trained for the site. If you build a hobby arm, keep fingers out of joints, keep the emergency stop in reach, and never run it unattended.
Where it goes wrong
Safe robot, unsafe tool. A cobot holding a knife is a knife.
Settings changed. Raising a speed or force limit to save time quietly undoes the assessment.
Missed sensor area. A person approaching from a blind spot is not seen.
Slow stopping. Heavier payloads and faster moves lengthen stopping time and distance.
Trapping. Even gentle forces can hurt if a person is pinned against a wall or table.
How real robots use it
Cobots are used for tasks like machine tending, screwdriving and packing, often with the robot running full speed when nobody is near and slowing as someone approaches. The mechanical side of the arm is covered in repeatability vs accuracy, and stop circuits are covered in emergency stops and safe robot design.
Try it yourself
Pick any machine you use: a printer, a 3D printer, a blender. List its hazards, rate each as low, medium or high for severity and likelihood, and write one change that would reduce each. You have just done a miniature risk assessment, and you will notice that the best fixes remove the hazard rather than add a sign.
Frequently asked questions
Can a cobot work without any guard at all?
Sometimes, if the assessment shows the application is safe with the cobot's built-in functions and nothing hazardous attached. That decision belongs to the risk assessment, not to the product name.
Why not just make the robot very slow?
Slower reduces risk but does not remove it, and a slow robot can still pin someone with a lot of force. Speed, force and tool shape are all part of the assessment.
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