Sensorless Robot Collision Detection and Hybrid Force/Motion Control

We consider the problem of real-time detection of collisions between a robot manipulator and obstacles of unknown geometry and location in the environment without the use of extra sensors. The idea is to handle a collision at a generic point along the robot as a fault of its actuating system. A previously developed dynamic FDI (fault detection and isolation) technique is used, which does not require acceleration or force measurements. The actual robot link that has collided can also be identified. Once contact has been detected, it is possible to switch to a suitably defined hybrid force/motion controller that enables to keep the contact, while sliding on the obstacle, and to regulate the interaction force. Simulation results are shown for a two-link planar robot.

I. Introduction

When a robot manipulator operates in an unstructured environment or shares its workspace with a human user, safety issues are of primary concern [1]. Main injuries may occur from an accidential collision between the robot structure and the environment (viz. humans), due to the uncertain location of obstacles and/or unpredicted relative motion. Avoiding such collisions requires (at least local) knowledge of the environment geometry and the use of computationally intensive motion planning techniques, see e.g. [2]. Anticipating incipient collisions or detecting them in real-time is typically based on the use of additional external sensors, such as sensitive skins [3], on-board vision [4], strain gauges [5], force load cells, and so on. When an impact occurs, the resulting contact forces may be alleviated, by pursuing a lightweight robot design [6], possibly with distributed compliant characteristics in the driving system and/or through a soft covering of the links [7]. Once a collision is detected, the controller should switch strategy and either stop robot motion or perform a more sophisticated interaction task.