Technical Bulletin: Understanding Power and Force Limiting in the FLR022-004 Cobot

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Overview

The NexBot Robotics FLR022-004 Collaborative Robot Arm is designed for safe, direct interaction with human personnel in a shared workspace. A core element of this design is its inherent safety system, which relies on advanced power and force limiting technology. This bulletin provides a technical overview of this system, how it functions, and its implications for application design and risk assessment. Unlike traditional industrial robots that require extensive physical guarding, the FLR022-004's safety features are integrated directly into its hardware and control architecture, enabling collaborative applications.

Principle of Operation: Power and Force Limiting

Power and force limiting is a safety function that ensures any potential contact between the robot arm and a person occurs at a level that does not cause harm. This is achieved by continuously monitoring and controlling the forces exerted by the robot. The system in the FLR022-004 operates on two primary principles:

1. Force Limiting: Each of the six joints on the FLR022-004 is equipped with high-resolution torque sensors. These sensors provide real-time feedback on the torques being applied by the motors and any external forces acting upon the robot's structure. If the controller detects a sudden, unexpected increase in torque—such as that caused by a collision with an object or person—it immediately triggers a protective stop. The sensitivity of this detection is configurable to match the application's specific risk assessment.

1. Power Limiting: In addition to monitoring force, the system also limits the maximum power (and therefore speed) of the robot's joints, particularly when operating in a collaborative mode. By capping the kinetic energy of the arm, the system ensures that even in the event of a failure in the primary force detection, the impact energy remains below established safety thresholds.

These two principles work in tandem. The robot operates within a safe power envelope, and the force sensors act as a constant monitoring layer to detect and react to contact events.

Key System Components

The effectiveness of the power and force limiting system depends on the tight integration of several key components:

• Joint Torque Sensors: Integrated directly into each axis (J1-J6), these strain gauge-based sensors are the primary data source for the safety system. They measure motor torque and external forces with high precision.
• Safety-Rated Motion Controller: The robot's controller is designed with redundant safety circuits. It continuously cross-checks sensor data and motor states. If any anomaly is detected, it can execute a Safe Torque Off (STO) or other category of safety stop.
• EtherCAT Communication Protocol: The FLR022-004 utilizes the high-speed EtherCAT protocol for communication between the controller and the joint servo drives. This low-latency, deterministic communication is critical for ensuring the robot can react to a collision event within milliseconds, which is essential for safety.

Implications for Application Design and Risk Assessment

While the FLR022-004 is designed for collaborative operation, a thorough risk assessment is mandatory for every application. The power and force limiting settings must be configured based on this assessment.

Key considerations include:

• Payload and Tool Center of Gravity (TCP): The robot's safety controller must be accurately configured with the weight, dimensions, and center of gravity of the end-effector and workpiece. Incorrect payload data can lead to inaccurate force calculations, compromising the safety function.
• Contact Scenarios: The risk assessment must identify all potential points of contact between the robot and personnel. This includes analyzing the nature of the contact (e.g., impact vs. clamping/crushing) and setting appropriate force, pressure, and speed limits in the robot's safety configuration.
• End-Effector Design: Sharp edges or pinch points on the end-effector must be eliminated. The design of the tool itself is a critical part of the overall collaborative application's safety.

By understanding and correctly implementing the power and force limiting features of the NexBot Robotics FLR022-004, engineers can create efficient, flexible, and safe automation solutions where humans and robots can work together effectively.

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