Power and Force Limiting (PFL)

PFL is not an add-on sensor; it is an inherent property of the robot's mechanical and control design. This is achieved through:

• Low-inertia actuators and lightweight links to minimize kinetic energy.
• Torque sensors in each joint to monitor and limit applied forces in real-time.
• Backdrivable gearboxes that allow the arm to be easily pushed away by a human. This design-first approach ensures safety is always active, even in the event of a sensor or software failure.

PFL limits are based on pain and injury thresholds defined in ISO/TS 15066. The standard provides maximum permissible values for different body regions, distinguishing between:

• Transient Contact: Brief, impact-like contact (e.g., a bump). Limits are higher, focusing on peak force and pressure.
• Quasi-Static Contact: Prolonged clamping or trapping. Limits are much lower to prevent ischemia and tissue damage. For example, the limit for transient contact on the forearm is 150 N, while for quasi-static contact it is 75 N. These values are derived from biomechanical studies.

A PFL robot continuously performs an implicit risk assessment based on its own state. The safety controller evaluates:

• Tool Center Point (TCP) speed and position relative to the workspace.
• Kinetic energy calculated from mass and velocity.
• Contact force and pressure measured at the joints. The system dynamically adjusts its monitored stopping distance and compliance to ensure that if contact occurs, the transmitted energy stays below the biomechanical limits, regardless of the robot's programmed task.

PFL is one of four collaborative operation modes in ISO 10218-1/ISO TS 15066. It is often used in combination with:

• Safety-Rated Monitored Stop: The robot stops when a human enters the workspace but does not limit its inherent power.
• Hand Guiding: The robot enters a zero-gravity mode for direct teaching.
• Speed and Separation Monitoring (SSM): Uses external sensors to maintain a protective separation distance. A typical collaborative cell might use SSM for approach and PFL for the final collaborative task phase, creating a layered safety architecture.

Verifying PFL compliance requires rigorous physical testing, not just simulation. According to ISO/TS 15066, this involves:

• Using a force-pressure measurement device to simulate human body regions.
• Conducting transient contact tests by moving the robot at maximum speed into the measurement device.
• Conducting quasi-static contact tests by commanding the robot to push against the device. All tests must be performed at the point of greatest possible force (usually the TCP with the heaviest approved tool) to certify the worst-case scenario is safe.

PFL is the enabling technology for Physical Human-Robot Interaction (pHRI), where contact is not just possible but part of the task. Key applications include:

• Cooperative Carrying: Human and robot lift an object together, with the robot complying to the human's lead.
• Assisted Assembly: The robot presents a part, and the human pushes or guides the arm into final position.
• Tactile Feedback Tasks: The robot performs a task like sanding or polishing, where the human may need to adjust the tool's pressure by hand. This transforms the robot from an isolated automaton into a compliant, responsive team member.

This involves loading and unloading parts from CNC machines, injection molders, or presses. A PFL robot can safely operate in the same space as a human operator who performs quality checks, deburring, or tool changes. The safety mode allows for:

• Close-quarters operation when the human needs to access the machine.
• Safe recovery from collisions if the human enters the workspace unexpectedly.
• Seamless hand-off of raw materials and finished parts, increasing line flexibility and utilization.

PFL robots are used to manipulate products or sensors for inspection. A human can physically guide the robot's sensor (e.g., a camera or probe) to a specific area of interest (kinesthetic teaching), and the robot can then autonomously repeat the scan. In material testing, the robot can apply controlled, limited forces for destructive or non-destructive testing (e.g., button press testing, flex testing) with the operator present to load samples and evaluate results in real-time.

In end-of-line operations, PFL cobots collaborate with humans to handle irregular, fragile, or high-mix items. The human might orient complex items or build mixed pallet patterns, while the robot handles the heavy, repetitive lifting and placing. The force-limiting capability is critical for handling deformable packages (e.g., bags, boxes) without crushing them and for safe operation in spaces where people are manually loading trucks or applying labels.

In research labs (pharmaceutical, life sciences, chemistry), PFL robots safely work alongside scientists. They automate repetitive tasks like:

• Liquid handling and pipetting.
• Microplate manipulation.
• Weighing and dispensing powders. The safe contact force allows a researcher to interrupt a protocol, manually adjust a sample, or correct a misaligned vial without triggering an emergency stop, maintaining workflow continuity and protecting expensive samples and equipment.

PFL is a foundational technology for robotic rehabilitation devices. Exoskeletons and therapy robots use force-limited actuation to provide assist-as-needed support for patients relearning motor skills. The system can safely apply therapeutic forces for gait training or upper-limb movement while ensuring it never exerts force beyond the patient's pain or injury threshold. This enables close physical interaction essential for effective, adaptive therapy.