Power and Force Limiting (PFL)

PFL is achieved through inherently safe design principles, not just reactive software controls. This includes:

• Force-limited joints with back-drivable actuators and mechanical compliance.
• Rounded, padded surfaces and absence of pinch points to minimize injury risk during contact.
• Low effective mass and inertia, ensuring the robot cannot transfer hazardous energy upon impact. The design ensures safety is maintained even in the event of a control system failure.

PFL systems are governed by pain and injury thresholds defined in ISO/TS 15066. The standard provides maximum permissible values for:

• Transient contact: Short-duration, quasi-static contact (e.g., a bump). Limits are defined for different body regions (e.g., forehead, hand, shin).
• Quasi-static contact: Prolonged clamping or trapping. Force and pressure limits are significantly lower. These limits are based on biomechanical studies and are the absolute foundation for setting a robot's power and force parameters.

PFL is enforced in real-time by the robot's control system, which continuously monitors and limits:

• Joint torque and current: Directly correlates with applied force.
• Speed and momentum: Power is a function of force and velocity; limiting both is essential.
• Contact detection: Using built-in joint torque sensors or motor current measurements to detect unexpected external forces indicative of human contact. Upon detection, the robot must enter a protective stop or reduce its energy to safe levels.

PFL is one of four collaborative operation safety modes per ISO 15066. It is often used in conjunction with:

• Hand Guiding: The robot is back-drivable under PFL, enabling safe physical guidance for programming.
• Speed and Separation Monitoring (SSM): PFL acts as a last line of defense if the protective separation distance is breached.
• Safety-Rated Monitored Stop: The robot stops before human entry; PFL enables safe resumption of work in close proximity. A true collaborative robot seamlessly transitions between these modes based on sensor input.

Deploying a robot in PFL mode requires a formal risk assessment and physical validation. This process involves:

• Identifying all potential contact scenarios (e.g., collision, clamping, guided movement).
• Measuring transmitted forces and pressures using a force-pressure measurement device as specified by the standard.
• Verifying that measured values are below the biomechanical limits for all tested points on the robot's surface and across its entire workspace. Compliance is not assumed; it must be empirically verified for the specific application.

While based on fixed limits, PFL implementation is not one-size-fits-all. Key tuning parameters include:

• Payload and tooling: The end-effector mass and geometry drastically change collision dynamics. Limits must be validated with the full tool attached.
• Task and trajectory: High-speed motions or movements near obstacles require more conservative settings.
• Collaborative workspace layout: The proximity of humans dictates whether PFL is the primary or secondary safety layer. Proper tuning balances safety with operational efficiency.

PFL cobots are integral to light assembly tasks where a human and robot work on the same component. The robot can handle repetitive, precise operations like screw driving, part insertion, or applying adhesive, while the human performs complex wiring or final inspection. The force limits ensure that incidental contact during hand-offs or close-quarters work does not cause injury.

• Example: A cobot presents a circuit board while a worker manually attaches connectors.
• Key Benefit: Combines human flexibility with robotic consistency without safety fencing.

In machine tending, a PFL-equipped robot loads and unparts parts from CNC machines, lathes, or injection molding machines. The human operator can safely enter the shared workspace to perform quality checks, clear jams, or change tooling while the robot is operational. The robot's inherently limited joint torque prevents crushing injuries if a worker's hand is near the gripper or a part.

• Example: A cobot retrieves a machined component, allowing a technician to measure critical dimensions in-cycle.
• Key Benefit: Maximizes equipment uptime by eliminating full safety shutdowns for human intervention.

PFL robots handle final packaging, case packing, and low-speed palletizing in environments where logistics are dynamic. They can collaborate with human packers who place irregular items or perform last-minute customizations. The force limits are crucial when handling potentially fragile items or when human hands are frequently in the robot's operational envelope to adjust packaging or apply labels.

• Example: A cobot places filled boxes onto a pallet while a worker inserts promotional leaflets.
• Key Benefit: Enables flexible, low-volume, high-mix production lines without extensive reconfiguration.

Cobots in PFL mode are used to present parts to human inspectors or to perform non-destructive testing requiring human guidance. The robot can maneuver a component under a camera or sensor, while the inspector can physically adjust the part's orientation for a better view. The compliant control allows the human to easily guide the robot's arm, a form of kinesthetic teaching, to define new inspection points.

• Example: A worker guides a cobot-mounted camera to inspect weld seams on a large, irregular assembly.
• Key Benefit: Enhances inspection thoroughness by combining robotic precision with human visual judgment.

In research labs and pharmaceutical settings, PFL cobots automate delicate tasks like pipetting, plate handling, and instrument loading. Scientists can work adjacent to the robot, preparing samples or reagents, with the assurance that accidental contact will not break expensive glassware or cause harm. The smooth, force-limited motion is essential for handling biological samples without agitation.

• Example: A cobot transfers microplates between an incubator and a reader while a technician adds reagents.
• Key Benefit: Increases experimental throughput and reproducibility while maintaining a safe, interactive lab environment.

Applications like sanding, deburring, and polishing often require the tool (e.g., a sanding pad) to be in continuous contact with a workpiece with controlled force. A PFL cobot can perform the repetitive motion, while a human supervisor can frequently intervene to feel the surface quality, change abrasives, or hold the workpiece. The robot's impedance control (often underlying PFL) allows it to maintain a constant contact force safely.

• Example: A cobot sands a composite aerospace part; a worker periodically feels the surface and adjusts the robot's path.
• Key Benefit: Automates ergonomically taxing tasks while allowing for expert human oversight and adjustment.

A collaborative robot safety mode where the robot's speed is dynamically controlled to maintain a calculated protective separation distance from a human, ensuring the robot can stop before contact occurs. This relies on real-time perception systems like LiDAR or 3D cameras to track the human's position and velocity.

• Core Principle: Maintain a minimum safe distance that accounts for robot stopping time and human intrusion speed.
• Contrast with PFL: SSM is designed to prevent contact, whereas PFL is designed for safe incidental contact.
• Application: Ideal for tasks where the robot and human work in adjacent but non-overlapping workspaces.

A robot designed from the ground up to operate safely alongside humans in a shared workspace. PFL is a core enabling technology for most modern cobots.

• Inherent Safety Features: Include force-limited joints, rounded edges, absence of pinch points, and smooth surfaces.
• Control Integration: PFL is typically implemented through series elastic actuators or torque sensors in the joints, coupled with software that limits motor current.
• Market Impact: Cobots have democratized automation by allowing flexible, close-proximity work in SMEs and complex assembly lines.

The subfield of HRI concerned with direct physical contact and force exchange between a human and a robot. PFL is a foundational safety requirement for enabling pHRI.

• Research Focus: Includes impedance control, admittance control, and physical collaboration where the human and robot jointly manipulate an object.
• Applications: Physical rehabilitation, cooperative assembly, and hand-over tasks.
• Safety-Critical: Demands rigorous validation against biomechanical standards like ISO/TS 15066 to prevent injury during intended or unintended contact.

A collaborative operation mode where a human operator physically grasps a robot's end-effector or dedicated handle and directly guides its motion. The robot's control system detects the applied force and moves compliantly.

• Activation: Typically requires a dedicated, safety-rated enabling device or button that must be continuously pressed.
• Programming Use: A primary method for kinesthetic teaching, allowing rapid task demonstration without code.
• Relationship to PFL: Often relies on the same underlying force/torque sensing and compliant control infrastructure as PFL to ensure safe, responsive guidance.

The simplest collaborative operation mode. The robot comes to a complete, safety-monitored stop when a human enters the collaborative workspace, and only resumes when the human leaves.

• Function: The robot acts as a static obstacle until the space is clear. This mode does not involve concurrent motion.
• Implementation: Uses safety-rated sensors (light curtains, area scanners) to detect human presence and trigger a validated safe stop.
• Use Case: Suitable for tasks where the human only needs intermittent access to the robot's workspace, such as part loading/unloading.