Abstract
In this paper, a new sensitive microwave lifedetection
system which can be carried by special rescuing robots
has been constructed. The system can be used for rescuing, antiterrorist
and law-enforcement purposes. The schematic diagram
of microwave Transmitting/Receiving (T/R) system with automatic
cancellation subsystem and signal processing system are
presented. The design is recited in detail. Experiments have been
conducted to verify the effectiveness of this system. The recorded
signal frequency spectrums for heartbeat and respiration of a
man behind an obstacle (wall) are presented.
Index Terms - Breathing and heartbeat signals, microwave lifedetection
systems, clutter cancellation.
I. INTRODUCTION
Existing ways for searching and rescuing human victims
buried under earthquake rubbles or collapsed building debris
are through the utilization of dogs, or optical devices. There is
great demand for constructing a new sensitive life-detection
system, which can precisely locate human beings under earthquake
rubble or collapsed building debris. The urgency of task
is defined by necessity for rescuers to begin disassembly of
rubbles and debris in places, where is a hope to find out the
alive people. Especially, this system can be used for detection
of the people and parameters of their moving inside of buildings
(or behind of the barriers) in course of antiterrorist operations
[1]. Generally, the radar that operates in the wavelength
range of 3 - 30cm (frequencies correspond to 1 - 10 GHz) [2]
could solve this task. In this case, by subtraction of signals reflected
from motionless objects it is possible to achieve high
sensitivity at detection of objects, borders of which are subjected
to mechanical fluctuations. According to estimation
available in the literature, the sensitivity of the method could
achieve 10-9 [2]. Recently, a microwave life-detection system
(L-band 2 GHz or X-band 10 GHz) has been developed [3]-[4]
to remotely detect the breathing and heartbeat signals of human
subjects at a 100-ft distance.
A series of experiments has been conducted to measure the
breathing and heart signals of a human subject through a layered
pile of bricks simulating rubble. It was found that an Xband
microwave beam cannot penetrate earthquake rubble or
collapsed building debris sufficiently deep to locate the buried
human victims [5]. X-band microwave became marginal when
the brick structure exceeded about 1.5 ft in thickness (about 5
layers of bricks), while for the L-band (2 GHz) system it was
possible to penetrate a pile of dry rubble of up to about 3 ft in
thickness (about 10 layers of bricks)[5]. This suggests that a
lower frequency will be more capable of detecting vital signals
of living subjects through a very thick barrier. Thus, for an
electromagnetic (EM) wave to penetrate deep (up to 10 ft) into
the rubble or the debris, the frequency of the electromagnetic
wave needs to be in the L or S band range [6]. For this reason,
our teams constructed a new sensitive microwave lifedetection
system, which operates at the L-band (2 GHz) for
special rescuing robots. The system is supposed to be sensitive
enough to detect the breathing and heartbeat signals of passive
victims who are completely trapped or too weak to respond to
the existing seismic detection system.
The basic principle for the operation of a microwave life detection
system for special rescue robots is rather simple. The
rescuing process is as follows. The controlled robot moves to
the danger field when commands are received through the
wireless channel. The system illuminate the human subject
with a low-intensity microwave beam (if a human subject is
buried, the microwave beam can penetrate through the rubble
or the debris to reach the subject) so that the small amplitude
body-vibrations due to the breathing and heartbeat of the human
subject will modulate the back scattered microwave signal.
It includes the breathing and the heartbeat signals. If the
reflected wave from the stationary background can be cancelled
[5] and the reflected wave from the subject’s body is
properly demodulated, the breathing and heartbeat signals of
the subject can be extracted. Thus, a human subject buried under
the rubble or the debris can be located.
To achieve a high sensitivity for this special life-detection
system, the clutter wave reflected from the rubble or the surface
of the ground has to be cancelled as thoroughly as possible
before it reaches the receiving microwave amplifier. Otherwise,
the clutter wave will saturate the receiving microwave
amplifier. In some existing systems, the clutter cancellation
was performed by a slow manual-adjustment process (by two
or more potentiometers). This is not practical for field rescuing
robot operation which demands a fast process. In our system, a
newly automatic clutter cancellation subsystem (that based on
the technology of microchip controller) to control programmable
microwave attenuation and a programmable microwave
phase shifter for performing the real-time automatic clutter
cancellation is developed . The basic principle of this subsystem
is to use the microprocessor-based control unit to scan the
attenuator and phase-shifter to minimize the input signal to the
microwave amplifier and cancel the clutter component.

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