Motion Planning and Control of Redundant Manipulators for Dynamical Obstacle Avoidance

Giacomo Palmieri,Cecilia Scoccia

doi:10.3390/machines9060121

Abstract

This paper presents a framework for the motion planning and control of redundant manipulators with the added task of collision avoidance. The algorithms that were previously studied and tested by the authors for planar cases are here extended to full mobility redundant manipulators operating in a three-dimensional workspace. The control strategy consists of a combination of off-line path planning algorithms with on-line motion control. The path planning algorithm is used to generate trajectories able to avoid fixed obstacles detected before the robot starts to move; this is based on the potential fields method combined with a smoothing interpolation that exploits Bézier curves. The on-line motion control is designed to compensate for the motion of the obstacles and to avoid collisions along the kinematic chain of the manipulator; this is realized using a velocity control law based on the null space method for redundancy control. Furthermore, an additional term of the control law is introduced which takes into account the speed of the obstacles, as well as their position. In order to test the algorithms, a set of simulations are presented: the redundant collaborative robot KUKA LBR iiwa is controlled in different cases, where fixed or dynamic obstacles interfere with its motion. The simulated data show that the proposed method for the smoothing of the trajectory can give a reduction of the angular accelerations of the motors of the order of 90%, with an increase of less than 15% of the calculation time. Furthermore, the dependence of the on-line control law on the speed of the obstacle can lead to reductions in the maximum speed and acceleration of the joints of approximately 50% and 80%, respectively, without significantly increasing the computational effort that is compatible for transferability to a real system.

Highlights

IntroductionRobots are increasingly asked to work in unstructured environments
Nowadays, robots are increasingly asked to work in unstructured environments.In many cases, they are supported by sensor-based safety systems, avoiding fences that typically isolate the cell workspace within the workshop
The first three examples help to understand the behavior of the control law in different conditions, with fixed or moving obstacles interfering with the manipulator in different points of the kinematic chain; a common set of parameters is used (Table 1) and a threshold θmax is imposed for the angular velocity of all joints

Summary

Introduction

Robots are increasingly asked to work in unstructured environments In many cases, they are supported by sensor-based safety systems, avoiding fences that typically isolate the cell workspace within the workshop. In addition to dedicated hardware and design principles, collaborative robotics implies specific control strategies to ensure safety [1,2] In this sense, collision avoidance control techniques represent a powerful means of improving the safety and flexibility of robots. A more complex problem is the collision avoidance for industrial manipulators, which suffer from the limitations of the workspace and problems of singularity In this case, redundant manipulators offer greater dexterity than traditional manipulators, which aids in the development of task-oriented control strategies taking advantage of the additional degrees of freedom. Redundancy can be exploited with standard manipulators if some of the degrees of freedom of the end-effector, e.g., the orientation angles, can be kept free during motion

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