Control of a Redundant SCARA Robot in the Presence of Obstacles
A redundant SCARA robot for operation in restricted work spaces is presented that performs collision free motion while simultaneously following a specified end-effector trajectory. The control system combines resolved motion rate control with secondary go
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W. Risse Gerhard-Mercator University, Duisburg, Germany M. Hiller
IMECB GmbH, Moers, Germany
Abstract A redundant SCARA robot for operation in restricted work spaces is presented that performs collision-free motion while simultaneously following a specified end-effector trajectory. The control system combines resolved motion rate control with secondary goals, in particular on-line collision avoidance. The implemented control algorithms are based on artificial potential fields induced by obstacles within a model of the work space. Results of experimental studies illustrate the manoeuvrability of the robot within an environment restricted by obstacles.
1 Introduction Robot manipulation in work spaces containing obstacles is usually limited by the manoeuvrability of the robot arm, especially for those tasks where a. prescribed end-effector trajectory has to be tracked. One approach to overcome such restrictions is to improve the robots dexterity by supplying additional axes. These redundant robots have the ability to A. Morecki et al. (eds.), ROMANSY 11 © Springer-Verlag Wien 1997
W Risse, M Hiller
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Figure 1: Redundant SCARA robot R.EDROB (top view) satisfy secondary goals in addition to the specified end-effector motion, e.g. to avoid physical joint limits, singularities or collisions with obstacles in their vicinity. In the following a planar redundant robot together with its control structure and algorithms is described with emphasis on its behaviour in the presence of obstacles. The manipulator under investigation is the redundant SCARA robot RED ROB that was developed and built at the lnstitut Jii.r Mechatronik. The mechanism has four revolute joints in parallel which give the planar system one redundant degree offreedom (DOF). It was mainly designed as an experimental setup for preliminary investigations of redundancy control and sensor concepts to be applied to larger redundant manipulators operating in construction or mining [1]. The modular design of REDROB allows the assembly of different mechanisms with varying length and dimension. In the configuration discussed (Fig. 1) the manipulator has a maximum reach of 1.54 m and can carry a payload of up to 6 kg. To detect obstacles located in the robot's vicinity, a configurable ultrasonic sensor system has been developed that consists of eight transducers on both sides of the three main links [2]. Since these devices have not yet been available, a model of the planar work space has been used to investigate on-line control strategies for operation in the presence of obstacles. In this context obstacles can be described as lines, rectangles, circles or combinations of these primitives. In addition the end-effector is equipped with two laser displacement sensors allowing the tracking of unknown surfaces with respect to distance and orientation.
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Control of a Redundant Scara Robot in the Presence of Obstacles
2 Control Structure The robot is controlled by means of a graphic
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