This paper presents the combined results of two studies that investigated how a robot should best approach and place itself relative to a seated human subject. Two live Human Robot Interaction (HRI) trials were performed involving a robot fetching an object that the human had requested, using different approach directions. Results of the trials indicated that most subjects disliked a frontal approach, except for a small minority of females, and most subjects preferred to be approached from either the left or right side, with a small overall preference for a right approach by the robot. Handedness and occupation were not related to these preferences. We discuss the results of the user studies in the context of developing a path planning system for a mobile robot.

Cognition in technical systems is especially relevant for the interaction with humans. We present a newly emerging application for autonomous robots: companion robots that are not merely machines performing tasks for humans, but assistants that achieve joint goals with humans. This collaborative aspect entails specific challenges for AI and robotics. In this article, we describe several planning and action-related problems for human-robot collaboration and point out the challenges to implement cognitive robot assistants. MotivationResearch on intelligent autonomous robots has mainly focused on machines that perform certain services for humans, such as robot butlers or autonomous cars [7,16,24,26]. The interaction in such systems is restricted to a user interface to give a task to the robot and when acting, such a robot has to ensure the safety of humans in its vicinity.A new class of robot applications takes a different view: a robot assistant performs tasks together with a human in order to achieve a joint goal. This approach combines the cognitive capabilities and manual dexterity of humans with the strength, endurance and accuracy of robots. Rosenthal et al. [20] use the term "symbiotic relationship" to describe the reciprocal complementation of skills.On the one hand, the combination of human and robot skills moves useful robot applications closer to the state of marketable products. Whereas a robot butler would be expected to prepare a cup of tea all by itself, a robot assistant might only be expected to bring the kettle and the tea for a person to prepare the tea herself. On the other hand, the close interaction and collaboration with humans requires a range of new cognitive capabilities for robots in order to coordinate the human's and robot's actions in a way that feels natural to people. This paper is concerned with the action side of humanrobot collaboration. This includes AI techniques for planning and plan execution as well as specific action primitives for navigation and joint manipulation tasks. Application ScenariosTo illustrate the idea of assistant robots, we present two application scenarios where a collaboration of a human and a robot can be of substantial help, even if the robot has restricted capabilities.