Power and Force Limiting: The Four Collaborative Modes

There are exactly four collaborative operation types, and only one of them permits the robot to touch a moving person. Safety-rated monitored stop, hand guiding and speed and separation monitoring all keep robot and human apart in time or space. Power and force limiting is the only mode in which contact is a designed-for event rather than a failure.

The four types side by side

Safety-rated monitored stop

The most under-used mode in the catalogue. The robot runs at full industrial speed while it is alone, and when a person crosses the boundary it comes to a monitored standstill with the drives still powered, so it resumes without rehoming. Any robot can do this, including a 500 kg payload arm. There is no speed penalty at all, because the human and the motion never coexist.

Practical notes: the stop must be a safety-rated function, not a program pause, and the boundary detection needs a rated performance level derived from the risk assessment. A stop category 2 keeps power on and is what makes the fast restart possible; a stop category 0 removes power immediately and costs restart time.

Hand guiding

The operator moves the robot directly through a hand-operated device with a three-position enabling switch. Releasing the switch, or squeezing it fully, stops motion. It is used for teaching, for lifting assistance and for fine placement of heavy parts, and it is the mode where the operator is deliberately in contact for the whole task.

The constraint is that the guiding device has to be at or near the end effector, and safe speed monitoring must be active so the arm cannot run away while a hand is on it. Hand guiding is rarely the whole cycle; it is normally one step inside an otherwise automatic sequence.

Speed and separation monitoring

The robot measures the distance to the person and scales its speed so that it can always stop before contact. The governing quantity is the protective separation distance, and it is the sum of four contributions: how far the human can travel during the reaction, how far the robot travels during the reaction, how far the robot travels while stopping, plus allowances for sensor uncertainty and position uncertainty.

The arithmetic bites. Human approach speed is conventionally taken as 1,600 mm/s. If the total system reaction time is 150 ms, the human contribution alone is 240 mm before the robot has moved a millimetre. Add robot stopping distance at speed, which on a mid-size arm at 1,500 mm/s can be 300 mm to 600 mm, plus scanner tolerance of 50 mm to 100 mm, and the monitored zone is comfortably a metre deep. In a cell that is 1.2 m wide, the robot is effectively crawling whenever anyone is in the aisle.

Power and force limiting

Here contact is allowed, and the design constraint is the energy the body region absorbs. Joint torque sensing detects contact within single-digit milliseconds and the controller reacts before the limits are exceeded. The permissible speed is derived from the contact model rather than from the motors, which is why the same arm runs at 1,900 mm/s toward a hand and 400 mm/s toward a chest at 20 kg effective mass.

What this mode demands is engineering rather than hardware:

• Every reachable contact scenario identified, classified as transient or quasi-static, and assigned a body region.
• Geometry designed for area rather than edges, since the pressure limit binds before the force limit in most clamping cases.
• Measured validation with a calibrated force and pressure instrument, repeated after any change to speed, payload or tooling.
• Pinch points designed out. A gap that can trap a finger is a quasi-static case, and quasi-static limits are half the transient ones.

Most real cells combine two

The types are not exclusive, and the standard expects a cell to switch between them within a cycle. A common and defensible pattern for machine tending looks like this: full industrial speed while the operator area is clear, speed and separation monitoring as the operator approaches the load station, and contact-permitted operation only for the final 200 mm of the handover. That keeps the fast segments fast and pays the collaborative penalty only where it buys something.

The mode changes themselves must be safety-rated. A cell that switches to contact-permitted limits under program control alone, without a rated function confirming the change, is a finding waiting to happen in an audit.

Frequently asked questions

Sources