research activities in 2007 - CSEM

RISE – The Rich Sensing Concept
A. Hutter, D. Beyeler, A. Brenzikofer, E. Grenet, F. Rampogna, L. von Allmen, C. Urban, P. Nussbaum
Within the RISE project a camera-based wireless sensor network for people detection and tracking purposes has been implemented. This sensor
network, which is based on three vision sensors and two 3D time-of-flight cameras, is an ideal test-bed for long-term operational tests and the
development of advanced algorithms. Furthermore, the installation is well suited for life demonstration purposes.
The multi-disciplinary project RISE targets the elaboration of a
heterogeneous sensor network for the purpose of people
detection and tracking within home and building areas. As a
result of the first project phase, which ended in 2007, a
demonstrator has been implemented in the CSEM entrance
hall. The demonstrator is based on two different camera
types: low power vision sensors [1] and 3D time-of-flight
cameras [2] . Vision sensors exploit the contrast information of
the observed scene and are distinguished by their huge
dynamic range of 100 dB as well as the low power
consumption of 80 mW. 3D time-of-flight cameras, on the
other hand, provide a three-dimensional representation of the
observed scene. Within the RISE project the vision sensors
are used to detect and track persons and objects whereas the
3D time-of-flight cameras are utilized in order to provide
additional height information of the detected objects. Further
essential components of the system are the wireless
communication link together with the data fusion entity. In this
article the basic concept together with the implemented interworking
of the cameras and the wireless system is described.
The data fusion algorithm and the related issues are subject to
a separate article [3] .
The demonstration test-bed covers an area of approximately
150 m 2 . The vision sensors operate with a 2.6 mm fish-eye
objective, which in turn provides a relatively large field of
vision, e.g. the area that is observed by one particular camera.
As such, the vision sensors have overlapping fields of vision
and cover the entire entrance area ranging from the entrance
over the two entrance side areas to the reception desk. The
field of vision of the 3D time-of-flight cameras, which require
active illumination, is limited to an area with a diameter of
approximately 2 meters. One 3D camera is positioned close to
the entrance whereas the second 3D camera is located right
in front of the reception desk. The network coordinator, which
acts as wireless data concentrator, is located in a closed box
with wooden shielding right under the central monitor in the
reception hall. An illustration of the disposition of the different
vision sensors and the 3D cameras is presented in Figure 1.
Figure 1: Disposition of the vision sensors and 3D cameras in the
CSEM entrance hall
A new sensor platform that is capable of hosting both, the
vision sensor as well as the 3D camera, and that provides the
required processing and communication resources has been
designed. The sensor platform includes a Blackfin 533 digital
signal processor running at 500 MHz, 2 MB Flash and 32 MB
SDRAM memory, an Ethernet connection for test and
debugging purposes and a hardware socket that connects
different wireless modules. For the purpose of the RISE
project the use of the 2.4 GHz ZorgWave module [4] was
selected, since the communication characteristics of the
module together with the associated protocol stack respond
ideally to the throughput and delay requirements of the
system.
The communication concept foresees that each sensor node
communicates the position data of each detected object
together with a time stamp and some additional object
information – in total around 100 Bytes – to the network
coordinator. Transmission at regular intervals (about every
100 ms) is mandatory in order to guarantee tracking
consistency. In addition to this regular traffic, specific data
requests (as for instance the transmission of the currently
observed image) should be possible. This results in a required
data rate of approximately 8 kbps for the regular traffic of each
sensor node plus some additional bandwidth for the irregular
data request traffic. In order to comply with these
requirements the IEEE standard 802.15.4 (which is identical to
the basic protocol layers of the ZigBee system) was selected.
The network operates in beacon-enabled mode with a beacon
interval of 123 ms and the guaranteed time slot (GTS) option
of the standard is used to transmit the regular traffic of up to
seven sensor nodes. The GTS option allows contention-free
channel access meaning that data packets are transmitted
with guaranteed throughput and delay. It should be noted that
the GTS feature, which is not a mandatory option in the
standard, was implemented in the CSEM K15 stack. The GTS
portion requires approximately 44% of the available
transmission time between network beacons so that
approximately 67 ms remain available to accommodate
irregular data requests. This corresponds to a sustainable
data rate of approximately 15 kbps in situations with multiple
simultaneous data requests.
The RISE project demonstrator is fully operational and can be
visited at CSEM upon request.
[1] S. Gyger, et al., “Low-power Vision Sensors”, CSEM Scientific
and Technical Report 2004, page 17
[2] T. Oggier, et al., “Miniaturized 3D time-of-flight Camera with USB
Interface”, CSEM Scientific and Technical Report 2002, page 37
[3] E. Franzi, et al., “Data Fusion for Wireless Distributed Tracking
Systems”, in this report, page 24
[4] CSEM Wireless Sensor Networks, www.csem.ch/wsn
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