What Is Power and Force Limiting? PFL Safety in Collaborative Robotics.

What Is Power and Force Limiting?

Power and force limiting, or PFL, is an approach to collaborative robot operation that limits the mechanical effects of contact between a robot application and a person.

PFL can use robot design characteristics, safety-related control functions, or combinations of these measures to reduce contact hazards.

For example, a collaborative robotic arm may operate at a limited speed and force so certain foreseeable contacts remain within the limits established by the application's risk assessment.

PFL is not a guarantee that any robot can safely touch a person under all circumstances.

Key Takeaways

• PFL aims to limit the forces, pressures, and other mechanical effects associated with robot-human contact.
• The complete robot application must be evaluated, including tooling, workpieces, speed, and possible contact situations.
• A robot advertised as collaborative is not automatically safe for every application.

How Does Power and Force Limiting Work?

Imagine a robotic arm assisting a worker at an assembly station.

The robot moves a component toward a fixture.

During the operation, foreseeable contact between the robot system and the worker may occur.

A PFL design can use measures such as speed limiting, torque monitoring, and suitable mechanical construction to manage the resulting contact.

Some robots monitor estimated external forces and initiate a protective response when configured limits are exceeded.

Other designs use low-energy mechanisms or compliant structures.

The acceptable approach depends on the hazards and the safety functions required for the application.

Why Do Force and Pressure Both Matter?

Force describes the mechanical push or pull applied during contact.

Pressure describes force distributed over a contact area.

Consider the difference between pressing a flat padded surface against a person's arm and pressing the same force through a narrow metal edge.

The total force could be similar, but the smaller contact area can create much greater pressure.

This is why evaluating the robot arm alone is insufficient.

The end effector and workpiece may introduce sharp edges, pinch points, or concentrated contact forces.

What Are Transient and Quasi-Static Contacts?

Transient contact occurs when a person and moving robot briefly collide without the person becoming trapped.

Quasi-static contact occurs when a person can be constrained or trapped between the robot and another object, potentially allowing force to build.

The two situations create different biomechanical hazards.

The risk assessment must consider the foreseeable contact conditions and relevant body regions.

PFL vs. Speed and Separation Monitoring

PFL manages physical contact within defined conditions.

Speed and separation monitoring, or SSM, uses safety-related monitoring to maintain an appropriate distance between people and hazardous robot movement.

For example, an SSM application may reduce robot speed or stop movement when a person approaches.

A PFL application may allow certain contact during operation within established limits.

Different protective approaches can be combined when appropriate.

Can PFL Eliminate Robot Guarding?

Not automatically.

Some collaborative applications may operate without conventional fencing in selected areas.

Others still require guarding because of sharp tools, heavy loads, dangerous processes, or other hazards.

The complete application must determine the necessary protective measures.

Frequently Asked Questions

Does a built-in collision sensor make a robot PFL-safe?

No. The required safety performance and contact limits must be evaluated for the complete application.

Can PFL be used with industrial robots?

Yes, when the robot system supports the necessary protective capabilities and the application is appropriately designed and validated.

The Bottom Line

Power and force limiting supports certain forms of human-robot collaboration by controlling the mechanical consequences of contact. Its effectiveness depends on the complete application and a properly established safety design.