Page 327 - Mechatronics for Safety, Security and Dependability in a New Era

Ch63-I044963.fm Page 311 Thursday, July 27, 2006 12:08 PM
12:08 PM
Thursday, July 27, 2006
Page 311
Ch63-I044963.fm
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detance between base stationstm] propagatijn loss Pt/Pr[dB]
(a) (b)
Figure 1: (a) Relationship between the distance and the propagation loss with presence of a human
body, (b) Relationship between distance and variance of propagation loss
Hardware system
The present system consists of base stations and a mobile unit. One of the base stations is connected to
a laptop PC, which serves as a position calculation server. Other base stations and a mobile unit are
composed of a Bluetooth module and a microcomputer. Size of the base station and the mobile unit are
x
x
both 60 103 28[mm], and their weight is 130[g](including a dry cell battery) as shown in Figure 2 (a).
First, the mobile unit establishes a communication link with every base station. Then the mobile unit
measures the transmission power to each base station, and the corresponding base station measures the
received signal power. The position server collects these data through communication links among the
base stations and calculates the mobile unit's position using the method described in the previous
section.
Bluetooth H8

Figure 2: Positioning system hardware
(a) Mobile station and base stations (b) base station connected to the position server

EXPERIMENT
The positioning system described in previous section was applied for tracking workers' positions in a
shipyard, because workers' positions and movements in the work area are useful for safety analysis and
task management. Since the arrangement of the work area changes frequently in shipyards, the
proposed system has merits over other positioning methods because of its simple and easy installation.
x
Six base stations were installed in a measurement area of 15 20[m]. A worker carried the mobile unit.
Actual position was recorded by visual observation. In this situation, RMS_error was 3.4[m].
Since the accuracy of the present system decreases as the distance between the mobile unit and the base

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