Abstract
In a human–robot collaboration scenario, operator safety is the main problem and must be guaranteed under all conditions. Collision avoidance control techniques are essential to improve operator safety and robot flexibility by preventing impacts that can occur between the robot and humans or with objects inadvertently left within the operational workspace. On this basis, collision avoidance algorithms for moving obstacles are presented in this paper: inspired by algorithms already developed by the authors for planar manipulators, algorithms are adapted for the 6-DOF collaborative manipulators by Universal Robots, and some new contributions are introduced. First, in this work, the safety region wrapping each link of the manipulator assumes a cylindrical shape whose radius varies according to the speed of the colliding obstacle, so that dynamical obstacles are avoided with increased safety regions in order to reduce the risk, whereas fixed obstacles allow us to use smaller safety regions, facilitating the motion of the robot. In addition, three different modalities for the collision avoidance control law are proposed, which differ in the type of motion admitted for the perturbation of the end-effector: the general mode allows for a 6-DOF perturbation, but restrictions can be imposed on the orientation part of the avoidance motion using 4-DOF or 3-DOF modes. In order to demonstrate the effectiveness of the control strategy, simulations with dynamic and fixed obstacles are presented and discussed. Simulations are also used to estimate the required computational effort in order to verify the transferability to a real system.
Highlights
As robot technology has advanced, new types of robots with improved safety functions have been developed
Since collaborative robotic systems should be configured to assist the operator during their work, they are often equipped with sensor systems able to perceive the human motion and to detect the position of objects inside the workspace that may generate a risk of collision with the manipulator
Starting from results obtained in previous studies [26], in this paper, the algorithms for collision avoidance are extended and implemented for Universal Robots manipulators; Machines 2021, 9, 113 the UR5 robot is used as test case, but the implementation to other UR manipulators is straightforward due to the identical kinematic structure, except for the lengths of the links
Summary
As robot technology has advanced, new types of robots with improved safety functions have been developed. In addition to the motion imposed to the end-effector, a repulsive velocity vector can be applied to the point of the robot that is closer to one of the obstacles, adding a task to the control system [18,19,20,21,22,23,24,25] This kind of approach was studied by the authors in [26,27] for a planar case; in addition to the standard method, a modified repulsive velocity method was introduced, which depended on the velocity, in addition to the position, of the obstacle, improving the capability of the algorithm to compensate for moving obstacles. Results and insights on future works are discussed in the conclusion (Section 4)
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