In recent years, the industry shifted from mass production to mass adaptation, so the need for partial automation of the industrial process overcame full automation. Partial automation covers the cooperation between a robot and a human in the production of a product. In such applications, the safety of workers who share a workspace with a robot becomes a significant problem. Several standards have been issued in recent years to ensure that intentional or unintentional contacts between the operator and the machine are as safe as possible. These documents direct a detailed risk analysis of each developed collaborative application. The analysis is used to identify potentially dangerous contacts where contact force must be evaluated. The measured values must be lower than the permissible limits given in the standards to ensure safety. To date, such measurements have been performed with a rigidly mounted measuring system. These types of measurements do not consider the mass and natural reactions of the free segment of the human body after impact.
The initial proof-of-concept study was based on a standard ISO/DIS 21260 that was later deleted. In this study, we assessed impact forces measured by a force sensor mounted on linear rails. The force sensor was freely movable in one direction. In the experiment, different weights were added to the measurement system mimicking different dynamics of human body parts. Results showed a statistically significant correlation between robots velocity and mass of the measurement system and impact forces.
The new edition of ISO/DIS 10218--2:2021 covers a transient collision measurement system, but this measurement method is currently not in use. We improved the existing measurement system used in the previous study by replacing the force sensor with a 1D measuring force cell with a piezo crystal element for a higher sampling frequency. We replaced the existing rails that used sliding bushings and used ball bearings, thus reducing friction. In addition, we constructed a simple gravity compensation system to prevent unwanted movement of the measuring system while measuring impact in the vertical direction. We used calibrated springs and rubber pads of a commercially available system to ensure the most reliable measurements possible. We attached the new measuring system onto a robot to cover a larger part of the workspace and automate the measuring process. Robotic programs were designed in a simulation environment, which enabled us to export the constructed programs to the controllers of the robots.
We compared collisions in different points within the selected part of the workspace at different directions, the robot's speeds, and the mass of the measurement system. Results showed that the robot's speed and mass of the measurement system significantly affect impact forces, while the direction of impact and the impact point have a smaller effect. Based on the measured values, it is possible to assess the safety of the contacts at selected points in the work area. If the contacts at specific masses are within limits given in the standard, we can say that these contacts are safe. Contacts that exceed the limits must be prevented, or the robot's speed or power must be limited.
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