Power and Force Limiting (PFL) Cobots

Definitional Boundaries & Taxonomy

Power and Force Limiting (PFL) is the most advanced and highly sought-after modality of human-robot collaboration (HRC), strictly defined under ISO 10218-1 and ISO/TS 15066. Unlike other collaborative modes: such as Safety Monitored Stop (SMS), Hand Guiding (HG), or Speed and Separation Monitoring (SSM) which rely on keeping humans and robots physically separated: PFL explicitly permits and mathematically guarantees safe physical contact between the robot and a human operator during active operation.

The definitional boundary of a PFL cobot hinges on its intrinsic ability to detect external forces, absorb impact energy, and instantly mitigate kinetic transfer to a human body. A true PFL cobot is an inherently safe cyber-physical system designed to operate in a shared workspace without external safety fences. This is achieved through a combination of lightweight mechanical design, rounded geometries to maximize contact area (reducing localized pressure), and ultra-fast, high-frequency control loops (typically running at 1000 Hz or higher) that continuously monitor motor currents, joint torques, and external momentum. The fundamental taxonomy of PFL systems categorizes them by their sensing methodology:

1. Current-based PFL (e.g., Universal Robots): Relies on monitoring motor electrical current to estimate external forces.
2. Joint Torque Sensor-based PFL (e.g., KUKA LBR iiwa, Franka Emika): Utilizes physical strain gauges at every joint for high-fidelity force measurement.
3. Tactile/Skin-based PFL: Employs capacitive or resistive skins covering the robot's links.

Historical Evolution

The conceptual foundation of cobots was laid in 1996 by J. Edward Colgate and Michael Peshkin at Northwestern University, who patented a "device and method for direct physical interaction between a person and a general purpose manipulator." Early cobots were entirely passive devices, using unpowered continuously variable transmissions to guide payloads while the human provided the motive force.