Multi-Focal Visual Servoing Strategies

Multi-focal vision provides two or more vision devices with different fields of view and measurement accuracies. A main advantage of this concept is a flexible allocation of these sensor resources accounting for the current situational and task performance requirements. Particularly, vision devices with large fields of view and low accuracies can be used together. Thereby, a coarse overview of the scene is provided, e.g. in order to be able to perceive activities or structures of potential interest in the local surroundings. Selected smaller regions can be observed with high-accuracy vision devices in order to improve task performance, e.g. localization accuracy, or examine objects of interest. Potential target systems and applications cover the whole range of machine vision from visual perception over active vision and vision-based control to higher-level attention functions. This chapter is concerned with multi-focal vision on the vision-based feedback control level. Novel vision-based control concepts for multi-focal active vision systems are presented. Of particular interest is the performance of multi-focal approaches in contrast to conventional approaches which is assessed in comparative studies on selected problems. In vision-based feedback control of the active vision system pose, several options to make use of the individual vision devices of a multi-focal system exist: a) only one of the vision devices is used at a time by switching between the vision devices, b) two or more vision devices are used at the same time, or c) the latter option is combined with individual switching of one or several of the devices. Major benefit of these strategies is an improvement of the control quality, e.g. tracking performance, in contrast to conventional methods. A particular advantage of the switching strategies is the possible avoidance of singular configurations due to field of view limitations and an instantaneous improvement of measurement sensitivity which is beneficial near singular configurations of the visual controller and for increasing distances to observed objects. Another advantage is the possibility to dynamically switch to a different vision device, e.g. in case of sensor breakdown or if the one currently active is to be used otherwise. The chapter is organized as follows: In Section 2 the general configuration, application areas, data fusion approaches, and measurement performance of multi-focal vision systems are discussed; the focus of Section 3 are vision-based strategies to control the pose of multi-focal active vision systems and comparative evaluation studies assessing their performance in contrast to conventional approaches; conclusions are given in Section 4.