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3.2 System Overview
Figure 5,6 and Table 1 show the system overview and the specification of the proposed system. At the front part of the wheelchair, the stereo camera is
mounted to measure the user’s head position and orientation. Additionally, a laser range finder is attached for localization at the bottom of the wheelchair. To process data acquired from these devices, two computers are used. The user controls the wheelchair with wearing a HMD which has only left display, and watches both real scene and virtual object displayed on HMD simultaneously. 3.3 Measurement of Head Position and Orientation
To measure the user’s head position and orientation, we apply a system using stereo camera pair4). This system has following advantages: non-contact, passive, real-time, robust, accurate, and compact. Figure 7 shows an example of the result of the measurement process. The measurement accuracy of this system is approximately 2[mm] for head position and 1[deg] for head orientation. 3.4 Measurement of Position of Wheelchair
The wheelchair can recognize its position and surrounding environment using a laser range finder and a map. The map consists of sets of line segments. Figure 8 shows an example of the map. The map is prepared beforehand, and the initial position within the environment is given. In the current position, the wheelchair observes front scanning data of 180 degrees
from the laser range finder. Then, pseudo scanning data on the map at the previous position and orientation are generated. Based on the matching of these two scanning data, the wheelchair can detect its current position and orientation. The matching process utilizes a histogram-based method which is an extension of the matching method proposed by Weiss et al 3). The outline of the process is described as follows: 1. To calculate angle-histograms of two scans. 2. To find the angular displacement dµ between them which gives the highest correlation. 3. To rotate the map by dµ. 4. To calculate translation histograms of two scans for x and y directions. 5. To find the highest correlations dx; dy for x and y directions using histograms. 6. To translate the map by dx and dy. 3.5 Navigation Information
In our system, navigation of the user on the wheelchair is achieved by displaying objects and a directional arrow for annotation. Annotation objects consist of character strings and cones. They are fixed on virtual space and used for explanations of real scene or a sign of corners of a navigation route. Directional arrow is always shown in front of the wheelchair during the navigation. The direction of the arrow indicates the position of the next corner of the navigation route, and the length changes depending on the distance to the next corner. Page 1,2,3,4
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