ELSEVIER Robotics and Autonomous Systems 19 (1997) 259-271 Robotics and Autonomous Systems Hierarchical refinement of skills and skill application for autonomous robots Michael Kaiser*, Riidiger Dillmann University of lfarlsruhe, Institute for Real-Time ComputerSystems and Robotics, D-76128Karlsruhe, Germany Abstract One of the major goals in designing learning robots is to let these robots develop useful skills over time. These skills are not only related to physical actions of the robot, but also to the coordination of activities, communication with humans, and active sensing. Throughout this paper, the interdependency between these different kinds of skills is analyzed. For the case of elementary action skills and coordination skills, methods for inegration of skill application and refinement are developed. It is shown that this integration has the potential to support long-term learning and autonomous experimentation. 1. Introduction The complexity of tasks that autonomous manipu- lation robots as well as autonomous mobile platforms have to solve is usually tackled through task decom- position. Here, two characteristic modes of operation can always be distinguished: Model-based operation, including path planing or assembly planning and execution on the base of an a priori giveri and possibly continuously refined geo- metrical world model. This incldes also mission plan- ning, which might be based on a given model of the world's "semantics". Reactive operation such as collision avoidance or compliant motion that involves a direct coupling be- tween the robot's sen~;ors and its actuators. The next action of the robot is determined by the current sen- sor readings - possibly - their history, and the current goal. These operations will from now on be referred to as the basic or elementary action skills of the robot. They are complemented by basic sensing skills that * Corresponding author. E-mail: kaiser@ira.uka.de. allow for a targeted use of the robot's sensors in the framework of plan and mission execution. As modeling and planning makes only sense down to a certain level of abstraction, both modes of operation are usually combined. Then, elementary skills represent the interface between the planning and the control level in the robot's architecture. They also determine the basic operators available for plan- ning: Only if the robot is able to associate a symbolic operator with a sequence of actions that are possibly dependent on its perceptions, i.e., only if the robot can operationalize the operator by applying a partic- ular skill, using this operator on the planning level makes sense [18]. In addition, coordination skills are required to foster the efficient use of the available elementary sensing and actions skills. To realize elementary sensing and action skills re- qures to map perceptions to actions by means of a strategy that is goal-oriented. Several possibilities to encode such a strategy exist (Fig. 1). The "traditional" approach is the model-based one, which tries to deter- mine the skill's application conditions a priori and to 0921-8890/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0921-8890(96)01)055 -3